Lachinn

Lachefinn is the Het ship between Finn and Lachesis from Fire Emblem: Genealogy of the Holy War.

Canon
While Finn and Lachesis have no special interactions in-game, the Mitsuki Oosawa manga adaption gives them a romantic relationship.

Finn is originally introduced to Lachesis as her personal bodyguard, assigned to her by his master Quan, and immediately develops strong feelings for her. While attempting to set up a tent and failing, Lachesis is helped by Finn, who teaches her how to set up a tent properly. Later he also figures out about Lachesis' feelings for Eldigan, and admits to being jealous of those feelings.

After Eldigan's death, Finn finds Lachesis next to the coffin containing Eldigan's body just before it is to be buried. Finn tells Lachesis to take better care of herself, which Lachesis shrugs off and almost immediately asks Finn to kill her as well. An appalled Finn immediately slaps her and leaves the room, upset that she would even suggest that of him. Lachesis continues to sink into despair and begins to isolate herself, while Finn, desperate to bring Lachesis out of despair, tries to get Lachesis to eat and tells her how worried everyone is for her, himself included. Finn is later teased about his apparent feelings for Lachesis by Ethlyn.

When Lachesis wishes for a white rose, Finn draws one on a paper, as White roses in the current season would be unable to grow. He later takes drastic measures to snap Lachesis out of depression, first by trying to force Lachesis to eat, then by stabbing himself with the Lands Sword while Lachesis was holding it, trying to illustrate that the reason Eldigan gave it to her was because he wanted her to live. The latter action mortifies Lachesis and causes her to heal Finn, alongside allowing herself to finally accept and move on from Eldigan's death.

Later, Lachesis decides to train to become a Master Knight, her promoted class in Genealogy, and trains with Finn in lances, growing even closer to him in the process. When she finds out that Finn will be returning to Leonstar with Quan and Ethlyn, Lachesis becomes upset over losing one of her closest allies, although she does not realize it and silently questions why she is suddenly so distraught. Finn later meets with Lachesis one final time while she is training, and one of the training sticks she is cutting flies off and hits Finn's hand, prompting her to wrap the wound in gauze. She also takes the opportunity to admit how hard it is to imagine that she might never see Finn again, as he was her closest confidante who "brought the color back to her world" and that she took Finn for granted.

She then asks Finn not to forget about her in Leonstar, to which Finn responds by confessing his feelings for her, saying that he can no longer lie to himself about them. The two of them then share a night of passion as Lachesis asks Finn what Leonstar is like, as Finn describes Leonstar and his wishes to take her with him, though he cannot due to his commoner status. As such, Finn promises to return for Lachesis once his status has improved, which Lachesis responds by saying she will wait for him. Lachesis later finds out that Finn left Lachesis with her first child, Diarmuid.

Fanon
Lachefinn is a very popular ship in the Genealogy fandom, partially owing to the fact that, in Thracia 776, Lachesis' daughter Nanna refers to Finn as "father", strongly implying that Finn and Lachesis were romantically interested in one another. While this has been debunked as Nanna is moreso Finn's adoptive daughter, Lachefinn remains a popular pairing in the Fire Emblem fandom; It is the most popular pairing for both Finn and Lachesis, with its only rival being Lacheowolf (Beowolf x Lachesis) on Lachesis' end.

There are theories that Nanna is Finn's daughter, while Diarmuid, Nanna's brother, is Beowolf's son. This is due to the fact that in Thracia Diarmuid starts out with the "Beo Sword," a sword strongly implied to be connected to Beowolf, as the only other person capable of equipping it is Fergus, who is rumored to be Beowolf's illegitimate child. This theory is further supported by the fact Nanna can't equip the Beo Sword, likely meaning Beowolf isn't her father.