Seiji Kishimoto is an anime design artist responsible for a great many anime character and production designs, including Elfen Lied. He is chiefly responsible for the design shifts witnessed when the series was adapted from the original manga, which seems to have won the approval of mangaka Lynn Okamoto, based on statements he made in the series notes.
He once worked at ARMS Corporation that was the main animator of Anime Elfen Lied. No further biographical information is available at this time.
With a large library of work, he should not be confused with an accomplished oral surgeon in Japan with the same name, nor Seishi Kishimoto, a mangaka with a somewhat similar name.
His IMDB listing