A gargantuan tusked turtle-like monster, Gamera has had different origins depending on which era he is involved in. In the original Showa Era, he has some absent connection to Atlantis, but not much in the way of details is presented; he is simply one of many monsters who roams the world and is frequently lured to do battle with other monsters. In the Heisei Era, he is a bio-engineered Super Soldier created by Atlantis after previous misuse of the same technology produced the horrors known as Gyaos, given life for the purpose of slaying Gyaos and surviving through the eons to ensure they are exterminated.
Glass Cannon: Discounting the original film (Then at his most obviously indebted to Godzilla, he's nigh-impervious), Gamera is amongst the most fragile of Kaiju; even light arms fire does him some damage (Admittedly not much, but considering Godzilla shrugs off an A-Bomb...). He does have a healing factor, but it's quite slow.
Most iconic of Gamera's foes, Gyaos are blood-sucking pteranodon-like kaiju with laser beams. They first appeared in the third Gamera film of the Showa era and were later revealed to have spacefaring relatives. In the Heisei era, they were presented as the arc-villains of the trilogy, having been accidentally created by the people of Atlantis and then gone amuck, necessitating the creation of Gamera to destroy them.
Arch-Enemy: To Gamera. At least in the Heisei series.
Bat out of Hell: The Showa era incarnation is a sort of giant bat. The Heisei era are referred to being giant birds, despite still retaining bat-like features.
Breakout Villain: Appeared in more than one Showa film, which led to their revival in the Heisei era and further cemented their iconic image. They even cameo in Gamera The Brave!
Vampire Bites Suck: In the Showa era, the bites suck since you could be killed by a single chomp.
Weakened by the Light: The Showa version doesn't like the sun, and in fact would die if exposed to the ultraviolet rays for too long. The Heisei ones also preferred night at first but after growing protective eye plates they could fly during the day without much hassle.
This is the skin of a killer FROM SPACE
A space-faring strain of Gyaos that appears in Gamera Vs Guiron, it is essentially a Gyaos that can fly from planet to planet under its own power.
Sea Monster: Can swim underwater, but from another planet.
The Swarm: Was originally just the leader of an invasion force with underlings using human disguises. Then those underlings combined with Viras for a Kaiju battle!
Angry Guard Guillotine
The titular monster of Gamera Vs Guiron, this horror is a bio-engineered Super Soldier created to protect the dying world of Terra from any and all assailants, predominantly the starfaring haemovores known as Space Gyaos.
Beware My Stinger Tail: Has a deadly stinger hidden in her tail she can use to implant her offspring in other monsters.
Dangerously Genre Savvy: Spends the entire film trying to destroy the one weapon capable of defeating her, impales Gamera's arms and knocks him on his back to disable him, and generally proved to be one of the most cunning foes Gamera has faced. In fact, she's the Gamera foe who came the closest to killing him in the Showa Era!
Disintegrator Ray: Her heat ray vaporizes flesh and entire city blocks. Doesn't have the same affect on Gamera, but still damaging nonetheless.
Face Full of Alien Wing-Wong: Implanted an egg into Gamera's lung, stunning him. It hatched in his lung and would have probably made its insides a meal if the heroes hadn't killed it.
Hell Is That Noise: In-Universe, her main weakness is an idol that makes a sound that's able to both weaken her and cause severe illness in humans. White noise is also discovered to be this to her. Her Disintegrator Ray also opperates on this.
Monster Is a Mommy: And she will use the nearest monster as the incubator for that egg. Gamera is free.
Sealed Evil in a Can: Was sealed away under the earth by the Devil Whistle thanks to it making a special sound which weakens her, keeping her in suspended animation.
Superpower Lottery: Can shoot quills that keep Gamera in place and take down jets, a massively destructive heat ray, the ability to suck opponents toward her, and impregnate opponents with her stinger tail.
Ambiguously Evil: Multiple possible explanations are given for his existence, some of which imply he was actually not inherently evil and was corrupted by Ayana's hatred. While he's undoubtly a dangerous monster and the film's main villain, it's unclear if he does the things he does out of a malicious nature or simply because he thinks its what he thinks Ayana wants him to do.
Woobie, Destroyer of Worlds: She lost her parents during the battle between Gamera and the first Super Gyaos, her adopted family openly talks bad about her, she and the one part of her foster family she cares about are bullied by the neighborhood. Yeah, she's directly responsible for Iris (and depending on which choice you pic from his Multiple Choice Past, perhaps directly responsible for what he became), but the poor girl's been through hell.