Kehaar is our favorite type of rabbit: the seagull type. Kehaar is the black-headed seagull that Hazel's rabbits befriend.
Kehaar shows up on their doorstep wounded and unhappy. But as the narrator tells us, these gulls are "gregarious", meaning social. So it makes sense for Kehaar to join up with the rabbits, even though he's, well, a bird. Kehaar becomes an unofficial member of the warren, promising to come back to visit in the winters (40.15). Which is a good thing since he's pretty much the entire air force for Watership Down.
Generally speaking, Kehaar is very useful for Hazel (and the story) because he finds out where other rabbits live (both at the farm and at Efrafa) and he helps fight off Woundwort during the raid. But he also comes in handy for two other reasons:
(1) He proves Hazel correct in his crazy idea to befriend non-rabbit animals. (But notice that this idea is both (a) inventive and (b) traditional, since there's a whole El-ahrairah story that Bluebell tells about this very idea, "The Trial of El-ahrairah.") This is useful because it shows us that Hazel is always right (as long as he's listening to Fiver).
(2) Kehaar is also a bit of comic relief, what with his Norwegian-ish accent that turns "mother" into "mudder," etc. (If you like accents, that's comic gold. Also, Richard Adams notes in his introduction to the book that he based Kehaar on a Norwegian resistance fighter.) That may be silly comedy, but in a book with this much blood and death, we'll take what we can get.