On his high school trip to Japan, as the dare while playing truth or dare, Romy ingested a piece of mermaid flesh, which according to Japanese legends grant immortality. 

Not only did it grant Romy immortality after a bout of diarrhea, this immortality made him survive the many alterations done to his body with his mind intact, giving him the base to become the king of monsters

Usually the parasyte will take over a person's head after devouring the host's brains. But because it entered Romy's penis while he used the public restroom, he managed to grab hold of the parasyte before it entered his torso, trapping it in his penis. Unable to enter his brain through his body, the parasyte devoured Romy's penis, replacing it while still serving the function of his cock.

In fact Romy is lucky this happened, allowing him to have a penis still proving he was a male, while inhabiting Rize's female body after his original body perished from the neck down. 

The parasyte is able to enlarge, change shapes, grow limbs and claws and grows eyes and a mouth to enable communication. It is sentient and while having a connection with Romy, feeling exactly like his own cock, it can also separate to move and act independently. 

After being decapitated and dismembered in an attack by another dioclonus' vectors, Romy fortunately was immortal due to his eating mermaid flesh, granting him immortality and was reassembled by Monster Cock. 

Somehow, Romy's left arm was instead bonded to Aiko's chimera arm, becoming a part of him. 

Later on, after Rize's body replaced his own, Monster Cock salvaged Aiko's Chimera arm which had been ripped up to replace his missing left arm. 

A  "Chimera" is a grotesque human-derived species that can drastically alter the shape of their own body parts.

In a freak occurrence, Romy's body smashed under a steel girder, Romy's decapitated head was attached to Rize's body whose head had similarly been smashed under a girder. 

Having being bonded to the ghoul named Rize's body from the neck down, Romy now has the female body of Rize the ghoul, together with her kagunes which are the main weapons of a ghoul on top of their strength and durability which far exceeds a normal human being. Somehow his chimera left arm which was originally Aiko's was bonded to his current female body which belonged to Rize and Monster Cock attached to his female groin, giving him a mismatched body indeed. 

A kagune is a ghoul's predatory organ and functions as their weapon and claws. It is usually as red as blood (the colors are varied in the anime to distinguish the kagune of each individual), flexible like the flow of water, but firm and sturdy. When released, a ghoul's physique is strengthened, they are more resilient, and their mobility heightens. A kagune is composed of Rc cells, which flow just like blood, can become as solid as teeth and can be described as "liquid muscles." The Rc cells are released from a kakuhou piercing the skin, and the released Rc cells form the kagune. The kagune are voluntary muscles as ghouls can control them whenever they want and repeatedly harden and soften them at will. Kagune size depends on the Rc cells' quality and quantity while the shape depends on the creativity and intellect of the user

Having faced the red man who stuck a red gem in his throat, Romy now possesses the Branch of Sin

The Branch of Sin sometimes abbreviated as BoS is the ability to control and freely move the user's own blood outside of their body as an extension of oneself. Those who possess a Branch of Sin are known as Deadmen.

The Branch of Sin can be formed into various shapes and objects and is often used as weapons. Their powers and shapes vary and are unique to every Deadman. This power was created by the Nameless Worm carried by the Red Diamond. To summon their power, they first need to inflict at least a small abrasion in order to draw the blood out of their body. The blood they extract appears as a sort of string that's directly connected to their veins and arteries, which can be restored afterward if it's still connected to them. In very few cases, the user can completely control their blood while it's not directly linked to them and can even reuse the blood to recreate their weapon/s. In a case where a Deadman sustains minor wounds, they can slow down their blood or halt it at certain portions of their body to prevent themselves from bleeding out.

Romy's BoS are long blood tentacles and purple hexagons that have incredible destructive power. He is also able to shoot bullets of blood whenever he is bleeding. Having never met a deadman, he does not know this abilities are called Branches of Sin

Having being bitten by a zombie, the bite apparently had no effect on Romy due to his mermaid flesh and other alien implants to his body, granting him regenerative abilities.

However, unnoticed by him, he gain enhanced strength and regeneration on top of a zombie's invulnerability to damage. He has no idea which abilities came from which body enhancements due to his body no longer being his original body, and all the many enhancements he received.

Romy was infected by a frenzied tadpole as he lay injured after fighting monsters

If a human becomes the host of a frenzied tadpole instead of fully-grown frenzied frog, the wild increase of desires and the transformation process of the host manifest slowly compared to regular fractured humans, this type of host is called a Fractured Human Warrior. This allows the host to use their mutated abilities without losing control of his/her self. These types of fractured humans are usually much more powerful than the regular type, due in part to them keeping their mental abilities and sanity, allowing them the higher potential to unlock more uses and forms of their abilities

Romy's frenzied human state turns his right arm into a pistol, rifle or cannon, looking part biological, part technology. This attribute can grow to cover his body if he so wishes

Romy was experimented on, injecting diclonus DNA in his body giving him the vector ability. Usually a diclonus can only created in females, but Romy, having a female body, is able to manifest as a diclonus

Vectors are invisible "arms," telekinetic in nature and origin, that most Diclonii appear to possess. They are a Diclonius' primary means of defense and, more commonly, offense against humans.

Vectors are stated to be made up of molecules, and the vibration frequency of these molecules determines their abilities:

• Low: can go through objects without causing physical interference
• Medium: lift and throw objects, disrupt blood vessels
• High: slash through most material of low density, carry liquid on surface
• Extremely high: visible, explosively powerful

Having been captured and experimented on by the Japanese Military, they injected Romy with Titan Spinal Fluid giving him the ability to transform into the Female Titan

The Female Titan is a 14 meters tall Titan. Having high agility and durability, the Female Titan can also generate boned at any part of its body, creating armor or weapons attached to its body. 

Having a strange dial attached under his belly button by one of the gigant crew, Romy gained the ability to grow to gigantic proportions, larger than even the Female Titan form. 

His strength and durability increased to match his size. 

The dial also gave him the ability to fly as well as shoot destructive beams from his hands

Taking a strange drink when he gave the alien a coin, Romy's body now can accommodate other person entering his vagina located under his testicles. 

While inside, their bodies merge and his body amplifies their superhuman abilities. 

Having been transported into another dimension, Romy was suddenly attacked, and found himself caught in the middle between the Oni and the Momotaru

Both used their weapons made of blood or bacteria respectively, and while Romy at first struggled, he found himself absorbing their abilities

The Bacteria lodged itself into his bowels through his ass and he found that he had gotten control of another weapon

As for the blood, as he already had weapons made of blood by his Branch of Sin and Kagune, he found yet another use of his blood, by being able to use them as weapons

Both abilities enabled Romy to use his own blood and the bacteria to form objects and weapons