Yumie is a katana-wielding Japanese nun. She is a fanatical Catholic and seems to genuinely enjoy killing people with her almost superhuman sword skills. She knows various Batto-jutsu (sword drawing) techniques and is adept enough to kill even from outside the room where her victim is, but is quite capable of smashing men's heads into walls with her bare hands as well. Yumiko has commented how hard it is for her to "get Yumie to sleep" after she has been provoked into action. Her personalities switch places, either by duelling for control inside of their mind, or a strong blow. Heinkel uses this technique to awaken the latent Yumie and have her brutally kill several terrorists in the first volume of the manga's extra Crossfire feature.
In Hellsing chapter 74 Relics, she attacks Walter, enraged after seeing him desecrating Father Anderson's remains. Although moving extremely swiftly, she is outmatched and cut into pieces by Walter's wires. Heinkel is hysterical and aims her guns at Walter, only to be shot and wounded by the Captain. Yumie makes an appearance in OVA 5 and will most likely make another in future OVA instalments.
The character of Yumie originally appeared in Crossfire as the violent alter-ego of Yumiko Takagi, a bespectacled and pacifist nun. Despite nearly identical character traits, the Yumie in Hellsing and the Yumie/Yumiko of Crossfire are two entirely separate characters, as Crossfire is a separate story and not part of the Hellsing canon.
Note: the Japanese characters for "Yumiko" (由美子) and "Yumie" (由美江) differ only on the last syllable. Many people tend to make the mistake of calling her berserker personality Yumi (with two syllables) instead of the correct name, Yumie (three syllables). Yumi (由美), Yumiko (由美子) and Yumie (由美江) are all different names. Yumi, however, forms base name for both Yumiko and Yumie. Yumi, as written, is composed of the kanji for "Significance/Reason" and "Beauty". The name 'Yumiko' adds the character for "Child" to the end (子 ko) — which seems to be metaphorically symbolic of that half of her personality, since she's innocent and childlike in her Yumiko form. The name Yumieadds the character for "Inlet" (江 e).
Yumiko is a shy, quiet nun who gets easily frightened, whereas Yumie provides raw, unfettered violence with her nigh-superhuman strength, fearlessness and fearsome katana skills that make her the ideal partner to Heinkel. Maxwell often berates the pair, not because of their excessive violence, but rather because their activities as assassins, mentioned as to have been dressed in the raiments of the Catholic Church, indicating a connection to the Vatican and by proxy to Iscariot, which is supposed to be a secret agency outside normal knowledge. She is also the only character of Japanese ethnicity in Hellsing.