No but seriously, Haseo is a solid merc. I don't like that his awakening stuff is suuuuuper high CD (Demon Awakening has the single highest CD of any ability that isn't variable I believe? And it's really not deserving of that much recharge), but WHO IS HASEO is a lovely ability and he's a solid merc. I feel you can probably do something better by giving him either Data Drain or Skeith as an ability, and I'm not 100% sure he's 5/week tier and not a high power 4 (quick question Ky Kiske or Alucard vs Haseo, who ya got?), but all in all he's fun, .Hack got rereleased with new stuff recently so he's current, and he adds something to the game. He deserves a place in it.
Should He Stay?: Yes.
Don't forget that Haseo got a new form in Last Recode. I take 5th form Haseo to stomp either Ky or Alucard. It's such a massive power boost that it goes from Xth form Haseo not even leaving a mark on Ovan's ice cage to 5th form breaking it fairly easily. And his new skills with it are just flat out overpowered, doing tons more damage than anyone else is capable of. 5th form is much closer to a 6 than a 4.
Excellent. Guess it's time for me to finally play .Hack now! ---