Kat is an important member of the Order in DmC: Devil May Cry, and serves as Dante's guide for much of the game. She is able to see into Limbo as well as project her astral form into that side-dimension, and is also familiar with several Wiccan spells.
She is the love interest of Vergil
Kat has a sweet, innocent nature, and is willing to risk her life in pursuit of the Order's goals.
Kat has strong feelings for Vergil, possibly explaining her zealousness for the Order, yet curiously, she chooses to protect Dante over Vergil before Mundus.
Vergil and Kat briefly fell in love, but Vergil wiped their memories in order to protect the two of them from what he perceived as a weakness. Some believe her loyalty to Vergil stems from the remnants of that love, although it is possible its roots lie more with her personal conflicts with the demons. The Chronicles of Vergil
Although she is not Dante's love interest, Kat does help him connect with emotions deeper than his initial rush of vengeance and bitterness, learning humility and compassion much in the same way the classic Dante did from his human side.
Through Kat, Dante sees the plight and struggles of humanity, and learns that his personal problems pale in comparison.