Gabvi

Gabvi is the friendship between Levi Ackerman and Gabi Braun from the Attack on Titan fandom.

Canon
Although the two were enemies at first, due to Gabi killing Sasha, Levi eventually forgives her, along with the other scouts. They form an alliance together due to the rumbling.

Later on, after Falco transforms into his titan and saves the scouts, Levi stays on Jaws with Gabi, to which she offers help, although Levi declines. When Gabi shoots the Okapi Titan, Levi is seen putting his arm on her, protecting her from the recoil of the gun.

Fanon
With Gabi being a minor, fans view this more as a father and daughter type of relationship. While Gabi and Levi's interactions were minor, fans have imagined Levi adopting Gabi or Gabi helping out Levi with his tea shop in an alternate universe. Since none of their interactions have been animated as of yet, there are no existing tags on AO3. There are a few fan arts with Levi being a father to Gabi.

Both Gabi and Levi are known to be soldiers/warriors, however Levi is much more stronger than Gabi as he's known to be humanity's strongest soldier, also due to his Ackerman blood. Meanwhile Gabi is known to be one of the best warrior candidates as stated, as Gabi is single handily threw a pack of explosives towards her enemies, which gave Marley victory at the end. Gabi's design is based of a female version of Eren Jaeger, it's most likely that Levi sees Eren in Gabi.