Seven (2004)

Blurb:
Nana (no-name in Japanese) has no memory of his childhood before the age of twelve, when a store owner took him in and cruelly named him. The only clues to Nana’s past life are the burn scars on his legs and his irrational fear of fire. Enter Mitsuha, a vagabond writer, who crashes Nana’s place at the insistence of a mutual friend. At first, Nana loathes Mitsuha’s very existence—although he can’t sleep alone, Nana doesn’t like it when his new roommate climbs into bed with him, either—but eventually, he warms to Mitsuha’s sincerity and kindness and finds happiness waking beside him. Can their budding relationship help Nana come to terms with his unknown past?

Source: www.anisearch.com/manga/1497