Yuu
Initially the two of them started off not as the best of friends but through trials and tribuations did they grew closer together. Can ulimately be seen in Book 7 where
Spoilers ahead for book 7 chapter 12
he included Yuu in his dream in that while the rest of the characters dreamt about their own ambitions, Ace was thinking of a way he and Yuu can travel between both Twisted Wonderland world and Yuu's original world.Deuce Spade
Rivals in almost every way yet despite that, they can be seen hanging out together.