Kokoro Belmont is now available as DLC, as well as her stage music and gamer profile pics.
She actually does play as a Belmont. Her default power ups include a Laser Whip, Holy Water, a Boomerang Cross, and an Ax. She can also replace some of those sub-weapons with a Knife or Boomerang, but those are attacks available to several other characters and might not be Castlevania inspired. If you've ever wanted to know what it'd be like playing a Gradius game with a Castlevania character, now's your chance.
I haven't played her too much yet. When she has a sub-weapon equipped, it is always used at the same time as firing her main laser, rather than by pressing a different button as in a Castlevania game. I think she can have more than one sub-weapon equipped at a time, which are all thrown simultaneously when she attacks. The laser whip is a continuous "flexible" beam attack that replaces her normal laser.
The music fits the stages pretty well. There's occasionally a couple notes from a Castlevania song, such as some Theme of Simon during boss battles.