Wolfox

Well, that's not exactly a lot of information about the units that you have available.. female Corrin is a buffer/debuffer, Male Robin is a counter against colourless units such as Takumi. And the "blue Pegasus Girl" is Shanna, right? She's a lance user and has decently high resistance which makes her a threat to red/blue magic users. And yeah, Desperation basically gives her a brave weapon effect when initiating combat, so it's a pretty powerful skill. Combine that with her Killer Lance+ and she'll procc skills like crazy as long as she's alive. The fact that her Cooldown skill only has three charges can make her dangerous.

I'd go with Male Robin, considering how common Takumi is used in the Arena. He's also pretty physically bulky, so taking a hit shouldn't immediately put an end to him.

That's just my two cents. Don't think I'll have to go bother going in-depth since there's a pretty decent website out there doing it for me: https://fireemblem.gamepress.gg/

thanks for the tip, and it was indeed Shanna. I think I'll use Robin on my main force and Shanna in a more offence based set, perhaps in combination with a potential Lon'qu for just skill trigger goodness