It is seen throughout the anime, that Kei is obsessed by the artworks that the Hikari's produced and he has an ultimate goal of contolling/obtaining the artwork "The Black Wings" - a living piece of artwork. Due to the power that the Hikari family possessed and the fact that this power was absorbed into their artwork, Kei's personality changed from the loving father to Satoshi, to a man that was almost abusive to him (he didn't seem to mind the pain Krad inflicted on his adoptive son.) However, the viewers only see Kei as an obsessive, power hungry man before the episode in which Mio Hio tries to remind Kei of who he really is.
He sent Satoshi back to middle school to spy on Daisuke Niwa and also hired the young prodigy to become Chief Commander of Police and in charge of capturing Dark even though that he always interferes. In the anime, he was the one who found the voodoo doll that was Mio Hio and sent her to the same school as Satoshi to make Daisuke wear a magical pendant in exchange of giving her a real human body - which ultimately failed.
Kei seems to have some sort of partnership with Satoshi's "other half", Krad. This is due to his desire to gain more power. He also doesn't seem to be particularly bothered that Krad often inflicts pain on Satoshi as well as knocking him out at one point to retrieve a certain piece of artwork from Satoshi's room.
Before the immense power of the artworks corrupted Kei, he cared a lot for his son and has been seen enjoying the paintings that Satoshi produces, it's hard for viewers to decide if Kei is a bad person or not due to the fact that power from the Hikari's art is controlling his mind and making him into a power-hungry, obsessive man. It is not made clear at the end of the anime if Kei actually died after destroying the Black Wings but it seemed that he had returned to his true self, as he ruined Krad's plan of destroying both Dark and Daisuke - and therefore freeing his son from Krad as he gets sealed away, with Dark into the Black Wings artwork. Although it might have also been that if he could'nt have the Black Wings he didn't want anyone else to either.