Which B!Celica should i build?

Id say +atk. It works really well with double lion and she has a lot of spd by default, and those units who match her spd will typically be dead in two hits anyway.

It depends on how you'd like to use her. If you are using the standard double lion db build, then I'd go with Atk. If you'd rather want a swordsman who exceeds any other hero in speed so that you double almost every time and get doubled very rarely, I'd go with Spd. Good luck!

+Spd. She needs all she can get so she doesn’t get doubled in high tier arena

You lucky devil😏 lol
+spd for more quads,but +atk can net more kills provided with spd buffs.

If you're playing L&D firesweep I might go +atk, actually... her spd is quite high to begin with; it's her atk that might have some issues when opponents get their defenses into the 40s and have enough HP to back it up.

+spd I generally consider better once Double Lion is down if you want her to fight on like a normal unit.