Toshiyuki Aoshima is the cousin of Sayoko Mishima and is a year or two younger than Keiichi and friends. He's a casanova and joins hands with Sayoko more than once in the earlier stories to try to rip Belldandy and Keiichi apart, or just to find out what Belldandy is.
He is often defined in the anime series by the sports cars that he and Sayoko drive. In Aoshima's case, an Enzo Ferrari this differs from the car he drives in the manga. He is also named after one of the Japanese scale model making company (Tamiya, Fujimi, Aoshima, Hasegawa, Otaki).
Although he is always surrounded by girls in the anime, he is shown as someone that is a playboy looking for the same ends, which often entail sex as an end result. The character created can be percieved as the anti-Keiichi, in the way that he tends to stand for everything opposite including his morals.
Although not really having that much of a role towards the latter of the anime series, he was essential in the beginning, as a way of testing Belldandy and Keiichi's love. He is first introduced to Belldandy in the anime, during episode 7 of the first series. Belldandy innocently asks Sayoko where she can fine a romantic place that she can give Keiichi his chocolate for the 14th of Febuary. Sayoko senses her naivity and sends her to ask Aoshima stating that he will know best place. We experience Aoshima's shady ways when he takes her to a hotel room and tries to advance on her, she is unwilling and uses her powers to get him off her. Ever since that first encounter, Aoshima becomes infatuated with Belldandy. He sees her as something that he can't have and therefore this fuels him to try and make her his. Later on in the series, he starts to suspect that something is not right with her, but he is not as vigilant as Sayoko in noticing she is actually Extra-Terrestrial until much later on. In terms of trying to break up the relationship between the two main characters, he tended in the end to bring out the best of their relationship and in some ways pushed them forwards in their progress (unwillingly of course).
His relationship with Sayoko Mishima is one that often seems rather distant and they only really work closely with each other as a means to break up the main characters relationship. This starts to falter later on in the anime as Sayoko starts to develope real feelings for Keiichi and therefore the outright malice behind her plotting dies down and so she no longer needs Aoshima's assistance in the matter. This relationship can be compared to the relationship between Megumi and Keiichi, and again one will notice that Keiichi's relationship with Megumi is one that is much more close and loving. By the end of the first season of the anime he has been punished (struck by lightning) by Urd so many times that the mere sight of her terrifies him.