A friend, pal, chum or buddy was someone who was more than an acquaintance but typically did not belong to one's family. The relationship between friends was called a friendship. Some friendships remained platonic, while others began that way, but led to romance.
In 2067, the deep space probe Friendship 1 was launched by UESPA from Earth with the intention of finding alien species. Information within Friendship 1 was offered freely with the hope that one day Humans would stand on alien soil and extend their hands in friendship. (VOY: "Friendship One")
Malcolm Reed initially resisted Captain Jonathan Archer's attempt to form a friendship with him, having been trained not to fraternize with superior officers. Archer told him that he once had a commanding officer who felt that way: "They're your crew, not your friends." He stated that he thought a lot about it when he took command, but the Enterprise NX-01's mission was not a typical one and they had only each other to depend on. (ENT: "Minefield")
In 2258 in the alternate reality, James T. Kirk discovered the Spock of the prime universe while marooned on the planet Delta Vega. Surprised that Spock knew his identity, he asked how this was possible and Spock replied "I have been, and always shall be, your friend." When he then stated that he was Spock, Kirk replied "Bullshit." (Star Trek)
In 2259, La'an Noonien-Singh noted that the Federation taught that if a way was found to empathize with an enemy then one day they could become friends. She considered this wrong, because she believed some things in the universe, such as the Gorn, were evil. (SNW: "Memento Mori")
Over time, James T. Kirk and Spock came to share a deep friendship. In 2285, Spock, dying of radiation poisoning after sacrificing himself to save the USS Enterprise, told Kirk that he had been and always would be his friend. He then gave him the Vulcan salute with the traditional accompanying phrase "Live long and prosper." (Star Trek II: The Wrath of Khan)
For much of Human history, the ceremony of breaking bread was a symbol of friendship and community. This tradition was less practiced in the 24th century. (TNG: "Time Squared")
Sylvia Tilly claimed a roommate was an automatic built-in friend. (DIS: "Context Is for Kings")
According to Jean-Luc Picard in 2364, close friends were quite rare for him, with two of his oldest and closest friends being the deceased Jack R. Crusher and Walker Keel. For Picard, friendship "must dare to risk", or it was no friendship at all. (TNG: "Conspiracy")
Data defined friendship as "I experience certain sensory input patterns, my mental pathways become accustomed to them. The inputs eventually are anticipated and even missed when absent". One of his friendships, formed in 2365, was that with the girl Sarjenka. Although Data helped to rescue the girl's planet, Benev Selec, from geological instability, Prime Directive issues meant that Doctor Katherine Pulaski had to remove all of the girl's memories of Data. Afterwards, Captain Picard told Data that remembrance and regrets were also a part of friendship and that understanding as much brought Data one step closer to Humanity. (TNG: "Pen Pals") That same year, during a hearing to determine his status as property or a sentient being, Jean-Luc Picard asked him why he kept a book that he had given to him. He described it as a reminder of friendship and service. (TNG: "The Measure Of A Man") Data considered both William T. Riker and Deanna Troi to be his friends. (TNG: "Time's Arrow") By 2367, his best friend was Geordi La Forge. That year, he wrote an extensive letter to Bruce Maddox, a man whose attempt to experiment upon him he once opposed, but later maintained a friendly relationship with. In the letter, he detailed his recent exploration of the topic of friendship, including his experimentation with friendly jibes and insults. He ultimately concluded that while there were many Human emotions he did not fully understand, he was not mystified by the desire to be loved or the need for friendship. (TNG: "Data's Day") Data and Worf also both considered each other to be a friend. (TNG: "Gambit, Part II") He also befriended the young boy Timothy, telling him that he had many Human friends. (TNG: "Hero Worship") Data also considered Worf to be a kindred spirit, finding a bond between them in the fact that they were both orphans rescued by Starfleet officers and that in many ways they were both still outsiders in Human society. (TNG: "Data's Day") In his last words to Jean-Luc Picard within a quantum simulation, Data told him that peace, love and friendship were all precious because of knowing that they cannot endure. In so saying, he asked, as a last request of the friendship between them, that Picard terminate his consciousness. (PIC: "Et in Arcadia Ego, Part 2")
In 2366, when asked by Deanna Troi why he had chosen to become Human as a punishment by the Q Continuum, Q answered that Jean-Luc Picard was the closest thing he had to a friend. Later, however, when the Calamarain came after Q, Picard stated that despite Q's protestations of friendship, his real reason for being on the USS Enterprise-D was protection. (TNG: "Deja Q")
That same year, Commander Riker and Lieutenant Commander Geordi La Forge came to Captain Picard with their concerns regarding Lieutenant Reginald Barclay, who had recently come to the Enterprise-D as a new diagnostic engineer. They recommended a transfer based on Barclay's performance, but Picard was concerned, not accustomed to seeing an unsatisfactory rating for a member of his crew. Picard, however, felt that this was too easy of a solution and ordered La Forge to try harder, telling him he was a member of his team. He told La Forge to make Barclay his best friend and when La Forge objected to this, he told him to put aside his personal discomfort. Later, in discussing Barclay with Guinan, he asked her if he had any friends and she told him he did not have any that she had seen. La Forge strove to follow Picard's orders and ultimately Barclay became a valued member of his team. (TNG: "Hollow Pursuits")
Guinan made fast friends with Ro Laren in 2368, with Jean-Luc Picard later commenting that Guinan was very selective about whom she called a friend. (TNG: "Ensign Ro")
An imaginary friend was one created entirely by an individual's mind. Such friends were often created by children who had difficulty for various reasons forming friendships. Both Clara Sutter and Guinan had imaginary friends, Clara's being a young girl like herself named Isabella and Guinan's being a Tarcassian razor beast. Even as an adult, Guinan sometimes still talked to her imaginary friend, though not nearly as often as she once did as a child. Sutter, however, endured a harrowing experience when an alien being assumed the form of Isabella. The alien unwittingly placed Sutter in danger, not realizing that the restrictions the adults around her had placed on her freedom were for her own protection. Finally coming to realize the truth, the alien left Sutter, admitting she had never had a friend before. The two expressed the wish that they would one day meet again. (TNG: "Imaginary Friend")
The friendship between Benjamin Sisko and Dax was one that spanned multiple lifetimes, due to the unique nature of Dax as a Trill symbiont that experienced the lives of multiple host bodies. As Sisko took command of Deep Space 9, he wrote in his station log that he was looking forward to a reunion with a very old friend, referencing Dax, currently in the form of Jadzia Dax, but whom he had previously known as Curzon Dax. (DS9: "Emissary")
The relationship between Benjamin Sisko and Kira Nerys aboard Deep Space 9 was initially antagonistic. Kira even went so far as to express her doubts regarding Sisko to Admiral Rollman, leading Sisko to warn her that if she again went over his head, he would have hers on a platter. (DS9: "Past Prologue") The two, however, soon developed a cordial working relationship. By 2373, nearly four years later, Kira replied that she considered Sisko a friend when asked by Kai Winn Adami if they were close. (DS9: "Rapture")
When Garak first met Doctor Julian Bashir, he told him that he was glad to have made such an interesting friend, even though Bashir suspected him of being a Cardassian spy. Not long after this, when the Klingons Lursa and B'Etor visited the station, Chief of security Odo began monitoring them as they were known renegades among the Klingon people. When Commander Sisko asked him if he had news about them, he commented that they made friends easily, noting that they were apparently making a business arrangement with Tahna Los, a member of the Bajoran terrorist group Kohn-Ma who claimed to have been reformed. (DS9: "Past Prologue")
In 2370, when Dax suggested that Lysia Arlin was interested in Odo romantically, he replied that it was ridiculous as women did not react to him that way. She asked if he was saying that he had never had a female friend and he replied that he considered Major Kira a friend. She clarified that she was talking about intimate friendship. Later, when talking with the holographically re-created girl Taya, she stated that if she was a Changeling, she would change shapes all the time and everyone would want to be her friend. Odo told her that it was not this simple, that when he was younger, people would ask him to change shapes and pretend to be his friend. They only wanted to see him become a chair or animal, but none of them really cared about him. (DS9: "Shadowplay")
The friendship between Tom Paris and Harry Kim was one of the first formed among the crew of the newly launched USS Voyager and one that would endure throughout the vessel's nearly seven years in the Delta Quadrant. This friendship began in 2371 on Deep Space 9 when Paris saved Kim from being conned by the Ferengi Quark into purchasing an overpriced memento. Later, after Kim learned the details of Paris's sordid past, Paris told him that everything he heard was true and that would be better for Kim to stay away from him because he wasn't exactly a "good luck charm." Kim, however, replied that he didn't need anyone to choose his friends for him. Later, when Paris rescued Kim from a shaft on Ocampa, Kim commented "Took you long enough" and Paris replied "How could I let down the only friend I've got?" To this, Kim quipped "What makes you think I'm your friend?" (VOY: "Caretaker") In 2373, both Paris and Kim were imprisoned in the Akritirian maximum security detention facility. A barbaric environment, prisoners fought over scarce resources and fitted with synaptic stimulators designed to induce edginess and jumpiness. At one point, Kim, driven to the brink of madness, very nearly killed Paris with a pipe. However, later when other prisoners tried to remove clothing from an unconscious Paris, Kim told them that Paris was his friend and that nobody was to touch him. Shortly after this, both he and Paris were rescued. Later, when Paris spoke happily to Kim of sharing a meal with him, Kim was shocked, reminding Paris that he had almost killed him with the pipe. Paris, however, replied "You want to know what I remember? Someone saying, this man is my friend. Nobody touches him. I'll remember that for a long time." (VOY: "The Chute")
Kes considered The Doctor, an Emergency Medical Holographic program, one of her closest friends and was disturbed when she learned that a memory fragmentation in his program might cause him to lose all memory of that friendship. (VOY: "The Swarm")
In 2374, when Captain Janeway of Voyager fell into a coma, Commander Chakotay was forced to take command of the vessel. He made the decision to break the alliance Janeway formed with the Borg. Afterwards, he told her comatose form that she was more than just his Captain, she was his friend, and that he hoped she would understand. When Janeway woke up, she was disturbed by Chakotay's decision, but the two ultimately reconciled, realizing that they needed to stop fighting each other in order to resolve the situation. Following this, as they discussed the rescued Borg, Seven of Nine, who remained on Voyager, Chakotay commented that she might not want to stay, given that she had been assimilated at a young age and the Borg Collective was all she knew. Janeway, though, felt that they had something the Borg could never offer her: friendship. (VOY: "Scorpion, Part II")
In 2375, The Doctor discovered evidence leading to a truth that had been hidden from him: that he had made the decision in triage to perform surgery which allowed Harry Kim to live, but sacrificed the life of Ahni Jetal. He was confused at first as to why this had led to a conflict between his ethical and cognitive subroutines, but then realized that he chosen to save his friend, something which he had not originally been programmed to do. (VOY: "Latent Image")
Aboard Voyager, The Doctor took Seven of Nine under his wing as a mentor, the two of them sharing certain similarities due to both having difficulties adapting to Human culture. The two came to respect and appreciate each other. He began giving her a series of social lessons. However, the rapport between the two of them was jeopardized when The Doctor brought Seven to a reception in the mess hall on a date and she discovered that he had made a wager with Tom Paris regarding whether or not she would arrive with a date. Afterwards, The Doctor apologized to Seven for his conduct, telling her that he had brought her to the reception because he enjoyed her company and that he felt they had grown closer over the past few days. He said that they had become more than colleagues, that they were friends, and she agreed with this statement. Secretly, however, he had developed romantic feelings for her, but he soon discovered that those feelings were not mutual when she told him that it had become obvious there were no potential mates for her aboard Voyager. (VOY: "Someone to Watch Over Me")