i am playing a Tiefling Rogue arcane trickster who is half Oni so he has a very heavy samurai/ninja feel. i have been toying around with the idea of getting a familiar, but i am having problems thinking of the right fit for a familiar that has the right feel. any ideas?

I read this, and thought fox. Don’t know why.

A rat that is learning kung fu from watching your character practice in the mornings and evenings?

I did a quick google search for "Japanese evil animal spirit", and the first thing that came up that was an actual animal and not like... an animal-themed humanoid is the "Nekomata", a cat with two tails who attack and kill people who mistreat cats.

A few things that aren't necessarily evil but fit in with a Japanese theme are things like Kitsunes, which are shapeshifting foxes.  There's the Tanuki, which is a real animal sometimes called the "Japanese Raccoon-Dog", and is considered a trickster. Hakujya is a white snake affiliated with a Buddhist Goddess who represents luck and wealth. There's also a lot of crow imagery in Japanese culture... such as the villainous Tengu who are usually associated with crows, or Yatagarasu, a Crow with 3 legs who served as a guide for Japan's first emperor.

Given that there's a relatively famous samurai rabbit, if your DM allows it I'd go almiraj

Quote from AntonSirius >>

Given that there's a relatively famous samurai rabbit, if your DM allows it I'd go almiraj

 
Or at least a hare remember, familiars can’t attack anyway.

If we're referencing Usagi, wouldn't the more appropriate pet be a tiny dinosaur? There's actually a vaguely scientifically accurate Velociraptor (in that it's described as turkey-sized and feathered, instead of the Jurassic Park version which is actually a Utahraptor) statted out that more or less qualifies for Find Familiar. Kind of high HP, as it's a 1/4 CR creature instead of 1/8, but without access to any of its attacks it's actually a fairly mediocre familiar aside from its survivability.