 If you liked
Kiseijuu: Sei no Kakuritsu
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 ...then you might like
Mecha-ude (TV)
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Let's see...
- Parasitic aliens who forged a bond with humans (Parasites and Mecha-Ude)
- Our protagonist (Shinichi and Hikaru) gets fused with a unique kind of their species (Migi and Alma)
- They fight some baddies (enemy Parasites and the Kagami Group & some rouge Mecha-Ude hosts)
Contrast is that Parasites are organic while Mecha-Ude are mechanical creatures. Also, the female lead has no powers in Parasyte while she does in Mecha-Ude.