Sweet, gentle nun Yumiko Takagi wouldn't hurt a fly. Trouble is, she has a split personality, and her other half Yumie would not only hurt a fly, she regularly slices and dices enemies of the Vatican.
Yumie's the assassin who partners with Heinkel. She wields a katana and uses the tougen battou jai ryuu (Island-plain drawn sword style) to, well, kill people. She's a berserker, going slightly crazy during battles; even outside of them she's short-tempered and excitable.
Yumiko is aware of Yumie's presence, and much prefers to have Yumie "asleep" rather than in charge. In the first Cross Fire Yumiko's been awake for a long while, but an emergency forces Yumie to be called on. When Yumiko's wearing her glasses, she's her sweet and lovable self; when they come off, Yumie comes out, and then all hell breaks loose.
In Hellsing, Yumiko has yet to appear at all; Heinkel and Yumie lead the Vatican forces into the battle. Heinkel's started to develop as a result. Whether Yumie or Yumiko will do the same - and whether they'll ever be reconciled - remains to be seen.
Why love Yumiko? Because she's a total sweetheart. Like Seras, she's a good girl with a dark side. Also, she's cute in glasses.
Why love Yumie? Because she's completely nuts, and batty people are always fun.