Allison was a very special Belmont because she has an extremely rare blood type (her blood is extremely sweet).
Her blood type allowed her to live as long as Alucard, she is quite shy and she is kind and beautiful.
But the only thing she valued the most was her childhood friend, Alucard.
The both of them were good friends at which Alucard refers to her as a "Beautiful Angel" until Alucard's mother was killed.
Allison was shunned the moment Lisa was killed so Alucard abandoned her. However, the only thing that Allison did not realize about Alucard was that even if he shunned Allison he is still in love with her.
Another reason why Alucard left Allison was because he might hurt her.
The two would meet again, much later, in the year 2035.
Allison took the alias of Yukie Sanada, while Alucard took the alias of Genya Arikado
The two (falling in love with each other) decided to guide and protect Soma Cruz on behalf of a mysterious government agency.