One of the more deadly and vicious members of the Batman Beyond Rogues Gallery is the one known as Inque. Inque was experimented on until she became the living ink she now resembles. She can be solid or liquid on a dime, which allows her to slip in and out of just about anywhere, making her a very good thief, but also a very deadly combatant.
However, much like ink, she can't get wet, or she'll break apart. It was with this knowledge that Terry and Bruce have been able to defeat her numerous times. Despite though, she always has a way of coming back.