Greta

Greta was Angel's closest friend. Greta presumably, like Angel, is from the second generation and probably grew up in Alphonse. She was killed by Angel with a stab through the chest when Angel awoke in her blood mist. 
The loss of Greta caused Angel to develop amnesia, most likely intentional, and forget about her claymore days. This also caused mental illness in Angel, causing her to go into blood mist at random times. In these episodes, Greta is always seen, indicating her deep presence in Angel's psyche and the heavy feeling of guilt Angel had killing her.
Currently, Angel doesn't know who Greta was but is constantly plagued by visions of Greta accusing her of killing her. This is because Greta, in her severed form, is mindlessly following Angel around, haunting her, and causing blood-mist episodes. She always appears slowly pursuing Angel, crawling over the floor, bloodied, and comes with the sound of screaming children.