|“||This sector is home to a very large creature the researchers call Serris. It is capable of moving and attacking at ultra-high speed. The more senseless and widespread destruction here may be attributable to Serris. No doubt the SA-X released it, but I can't be sure why yet. Serris has returned often to the breeding tank here. Its natural behavior must be to go back to it periodically. It's a valuable specimen, but you have been authorized to terminate it. If you don't, it may invade other sectors.||„|
Serris' habitat is located in Sector 4 (AQA). Serris is a long, aquatic, serpent-like creature. Adam describes it as a "very large creature" that is "capable of moving and attacking at ultra-high speed". Considerable damage was done to Sector 4 before Samus arrives; while the damage that was done to the sector that was seemingly coordinated and deliberate was likely committed by the SA-X, the more wide-spread and senseless damage is believed to have been done by Serris. There is a breeding tank in the sector which Serris often returns to; Samus must reach it in order to terminate Serris, despite it being a "valuable specimen".
Once Samus reaches the tank, she finds Serris's skeleton. She proceeds to the next few rooms and fights an X Parasite mimic of Serris, which is the true cause of the damage in Sector 4.
Serris' only attack is ramming Samus. Serris will jump out of the water and across the various platforms in the room; it occasionally jumps from one side of the room to the other. It will also undulate between the platforms. There are railings on the ceiling which Samus can hold onto to avoid the serpent. For the entire battle, Serris follows an attack pattern. For example, after undulating through the platforms, Serris will leap across the room and try to ram into Samus while she is on the railing. However, if Samus stays at the highest point of the ladder on the right, Serris will almost never hit her. Serris will simply rush by, narrowly missing Samus unless it jumps up from beneath at a certain angle. This relatively safe spot allows Samus to charge her beam and wait for Serris to jump in her direction so she can shoot.
In order to beat Serris, Samus must shoot its head, the only vulnerable part of its body, with Missiles or charged shots. Upon impact, Serris glows and begins moving much faster, temporarily rendering itself invincible. After a few seconds, it returns to its normal form.
After firing five Missiles or three charged shots, Serris will transform into a Core-X, which now requires 5 missiles to shatter its shell rather than the usual three. It is important to remember that its movement is not hindered by water, but the basic Fusion Suit is highly sluggish when submerged. Accidentally falling to the bottom of the pool may prove to be fatal. Once the Core-X's membrane is destroyed, Samus can absorb it to regain her Speed Booster ability, which was demonstrated by Serris during the battle.
- Like many of the boss creatures featured in Metroid Fusion, the X-mimicked Serris had an ability which greatly resembled one of Samus' Chozo items (in this case, Speed Booster). According to Adam, the ability to move at incredible speeds was a natural characteristic that the original Serris had prior to its death by X infection. This would imply that Samus absorbed this natural aspect of the creature to reactivate or replace her own Speed Booster as she touched the Core-X.
- The Serris (along with, presumably, its entire race) and Dachoras are the only known species to possess an ability similar to the Speed Booster besides the Chozo.
- The battle with Serris is similar to Botwoon in Super Metroid, Acid Worm in Metroid: Zero Mission, as both have hiding techniques, and their weak spot(s) are on or near the head. The Vorash in Metroid: Other M likewise has a habit of leaping out of liquid over the platforms Samus stands on during its battle.
- Serris' brain is visible underneath a clear plating on its head. This may be another reference to the Alien franchise, from which the Metroid series has drawn significant inspiration, as some Xenomorphs are depicted with the plating on the top of their head being transparent (and thus have a visible brain).
- Serris's Japanese name, Ishtar (イシュタル), comes from the Mesopotamian East Semitic goddess of fertility, love, war, sex, and power.
- Sector 4 has a breeding tank for the Serris, which may suggest that there may have been more of its kind onboard.
- Metroid Prime and Metroid Fusion: Prima's Official Strategy Guide refers to Serris as "the snake" in several instances on page 102.
- Serris shares its boss music with Yakuza.
|Tallon IV||Aqua Reaper • Aqua Sac • Chomper Fish • Fish • Jelzap • Tangle Weed • Tallon Crab|
|Aether||Blogg • Bloggling • Alpha Blogg • Fish • Brizgee • Dark Blogg • Grenchler • Hydling • Krocuss • Phlogus|
|Dark Aether||Chykka • Dark Blogg • Dark Grenchler • Dark Phlogus • Dark Shredder • Flying Ing Cache • Venom Weed • Watchdrone|
|Zebes||Botwoon • Bull • Cacatac • Choot • Draygon • Evir • Gadora • Gamet • Oum • Owtch • Powamp • Puyo • Sciser • Skultera • Tatori • Tatori, Jr. • Yapping Maw • Yard • Zoa|
|BOTTLE SHIP||Fish • Skultera|
|BSL||Aqua Zebesian • Evir • Gadora • Owtch • Powamp • Sciser • Serris • Skultera • Yard|