The Serris is a large sea serpent that was held in the aquatic environment of the B.S.L. Research Station, Sector 4 (AQA). It had became a host for the X Parasite after they breached Sector 4, and the parasites made use of its high speed to cause heavy damage to the upper portions of the sector.
As an X-host, the Serris's combat abilities were enhanced exponentially, and it was able to use high-speed attacks. Also, the Core-X of Serris possessed the Speed Booster function from Samus Aran's Power Suit, and it used this ability to effectively double movement speed for a short duration of time. Its body is protected by a tough carapace, but its head is vulnerable to high-power attacks. It activates the Speed Booster when it sustains damage.
- Metroid Fusion (First appearance)