The bad thing about his curse was that his power was quickly growing, and couldn't be contained within his child form. He had to get the curse to break if he was to live... and if Belldandy was to live.
Well, in the end, Belldandy finally speaks his name. However, Welsper then hacked into Yggdrasil, and was punished afterwards by being reincarnated into a cat.
Still, though, he cursed himself AGAIN in order to remember everything and keep his intelligence. So he still lounged around at Tarikihongan in cat form, but Hild found him, seeing his punishment living as a girl cat.
Also, after Welsper had the Demon Servant transferred over to him (from Keichii who also recieved it from Belldandy and Belldandy had it transplanted by Hild when she was passed out due to Angel Eater which got her angel: Holy Bell), he got white spots. In order to remove those white spots, he had to name his Demon Servant. After a long chase (Urd and Skuld chased him in order to hear the Demons Servants name) he had named her: "Die Wespe der Blauen Lanze" ("The Wasp of the Blue Lance" in German). After he did, he returned to his normal color.
Later, when Hild was overthrown by a rebellion caused by the 2nd strongest demon Hagall, her child form(which she left on Earth) became aware of the events and has asked for the help of the goddesses. The goddesses accepted but in order to enter Nifelheim (Hell) they needed the approval of three demons. Those were Hild, Mara and Welsper ( When Hild took Welsper by a portal to their location he didn't recognize who got him there and threatened the summoner but as soon as he saw Hild's cold, merciless look he accepted doing it).