The question is kinda vague. I'll try to fill in.
If playing your character alongside your friend's character is wat you want, it's easily arranged through asking GMs.
As for what a Yamataian is... it's a new bio synthetic android that both the Geshrin race and the Nekovalkyrja can have their minds 'soul-transfered' to. So, your character would probably have been someone else before becoming a Yamataian, and it's a good idea to pay attention to that.
The Yamataian body can be altered on creation to match the appearance of a neko or a geshrin, depending on the character's preference when he actually made the change. It can, unlike nekos, bear children and it's more suitable to civilian life - not to mention they have a much longer lifespan.
There are no nekovalkyrja males - or rather, they are, but it's about as normal as seeing two-headed dogs crossing the street. If you want a 'guy' with 'cat ears', your character could have been a geshrin whom wanted to make the body switch and decided to try out neko ears just for the fun of it - or something.
For more info on playable races, go here :
https://stararmy.com/forums/viewtopic.php?t=1874
useful little thread which indicates the ups and downs of the mainstream species in the SARP.
