Kyoushoku Soukou Guyver (2005), Kiseijuu: Sei no Kakuritsu Recommendations
Both titles are based on sci-fi manga from the 1980's. Both focus on the main protagonist who is merged with an alien power that gives superhuman abilities. The main protagonist in both anime use their powers to fight off other evil alien monsters disguised as humans and both kills their enemies with gruesome attacks. The differences between the two is that The Guyver is more superhero oriented while Parasyte is more psychological horror. If you like, you should enjoy the other.
These two anime have a young boy that gain superhuman powers from aliens. The main hero in these two anime have to fight other evil aliens while fighting their humanity. The main hero in both anime have share a relationship with their powers too.