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.

Parasyte

medium

Parasyte

medium

Parasyte

HIGH

Parasyte

HIGH

Lots of blood, some brutal death scenes, not for the faint of heart, but certainly not the most graphic anime either.

Parasyte

HIGH