Izumi is a shinigami who appears later in the story. He is somewhat cold-hearted and distant, particularly in the anime. Of all the characters in the series, Izumi is the only one whose personality and plot role are noticeably different in the manga and its anime adaptation.

In the anime, Izumi's background story is not mentioned and he is portrayed throughout as a cold-hearted character, often as the serie's main antagonist. He makes several attempts to steal Mitsuki's soul over the course of the series, and although he claims to love Meroko, he only seems to do things that cause her pain (as Takuto points out). It is also revealed later that he was the one who assigned Takuto and Meroko to Mitsuki in order to force Takuto to remember his past life as a human by taking the soul of the daughter of his former friend.

In the manga, Izumi is much different. He is not set as the stories antagonist, but merely as someone who has lost his path.

He did not do most of the antagonizing things he did in the anime, as he did in the manga.

In the manga Arina Tanemura makes him a main character who appears in the story in chapter 5. He eventually lives with Mitsuki and the other Shinigami. In volume 5, we find out his backstory.

He was often abused by his bereaved mother when he was 6, and, to make her happy, he threw himself in front of a train, seeing his mother smile, but never understanding what she meant by it. He later took his mother's soul, but was still unsatisfied. As the story progresses Izumi also becomes an important person to Mitsuki and an important person again to Meroko.

The two eventually get back together with the aid of Mitsuki.
(Source: Wikipedia)