Butterfly is the queen of battles as a Melee Assassin and fighter. She can carry games easily by dealing massive damage. If you are planing to play Butterfly, you can unlock her with Cheap Arena of Valor Vouchers at 5mmo.com and be sure to read this Butterfly In-Depth Guide to get better understanding of this hero.

 
Butterfly Overview
Butterfly, appropriately named "the death's whisper". The face of the game, and also the savior of all players suffering from ELO Hell. Say whatever you want about her but I can guarantee that people will still mention her as one of the best hero to solo rank with. This holds true even until Platinum, although you'll have a bit harder time in getting kills after reaching it due to how she needs to be played.
 
What is the best build for her?
Leviathan build is certainly the safest build for her right now,along with mostly tanky items.. It goes against the concept of an assassin, but her skills and playstyle demand such builds. You can't trigger the passive if you're dead, and you don't have the option of running away.
 
Is she the best assassin hero?
Hardly. I'd say she's probably middle of the pack.She's the best when you're only starting out or in lower ELO.She's truly a high-risk high-reward hero that rewards you for going all-in at the right time, and punishes you hard otherwise.
 
Is she really “OP”?
As OP as any hero could be in certain situations, I suppose, but not inherently OP. Play her carefully, and with a little luck you can have some pretty interesting results.
 
What Arcana should you use for her?
Typical physical jungler arcana, which are 10 Onslaughts-10 Guerrillas-10 Skewer,but Guerrilla also works well in place of Assassinate. It’s probably better if you do some experimenting first and make your own set.
 
Which heroes complement her in a team?
No specific heroes, but Butterfly needs a team that can reliably fend off the enemy team in a 4v5 situation. A team that could take a beating, and wear down the enemy team enough for you to swoop in and get slicing.Once the enemies are low, Butterfly then can finish them off with little to no difficulty. Finding this kind of team can be very difficult though. Ideally you’d want Plenty of combinations could work as long as they're not overly squishy, and your coordination's good.
 
General Gameplay Tips for Butterfly
Only take soulreaver if you trust your tanks and team to take damage and start a fight properly as she is squishy if you don't use Leviathan. 
 
Always take Gilded Greaves as boots, useful for the CC reduction, only take it if they have 0 mages.
 
You should usually take Frost cape as she has no CC and the cape helps her stick to enemies, but you can go for Omni for more damage. 

Last, remember to disengage when you know you can't kill the enemy right away, back off for a few second and w8 for you Ult to cool back down if you didn't kill them, also remember to save your 1st for escape