After revealing to Raiden that he is his adopted father, Solidus also makes the chilling revelation that he killed Raiden's biological parents so he can be trained as a soldier. The father-son reunion turns predictably violent as Solidus announces his intentions of killing his wayward son.
Similarly to Venom Snake, Raiden was trained to be another Snake (he even briefly wore the title before receiveing the codename "Raiden" due to terrorist cell's leader calling himself Solidus Snake). Not to mention that just like Big Boss created Venom Snake, his son Solidus Snake created Raiden.
Snake first meets Raiden during a mission on the captured Big Shell in Metal Gear Solid 2. This mission is especially important for Raiden as it's his first big assignment under what he believes to be FOXHOUND, the same special forces unit that Snake worked for.
As a prisoner, the Patriots used Raiden as a test subject for experiments in exoskeletal enhancement surgery, during which he had his head and spine excised from his body from his lower jaw down and subsequently grafted into an enhanced synthetic body.
The real reason why Raiden was called "Snake" is because he represents the player having taken the role of Solid Snake in previous games/VR missions/Tanker Chapter. Minor nitpick, but a solidus isn't a gas, it represents a point between solid and liquid states.
He's referring to the events of Twin Suns from Metal Gear Solid 4, where he had to lose his arms to save Solid Snake (specifically, he willingly cut off his right arm to save Snake from Outer Haven due to being otherwise unable to move in to aid Snake, and he lost the remaining arm when he was crushed by Outer Haven).
I remember seeing somewhere that Raiden was created and designed as a response to the first game not having a large female playerbase, hence his softer face and white hair compared to Snake's rougher look and darker hair.
White blood is an artificial blood initially developed for military use. Hemoglobin, which is found in red blood cells and absorbs and releases oxygen, is normally red. However, the artificial red blood cells developed through nanotechnology, and perform the same function as hemoglobin are instead white in color.
In MGS3, Snake starts the game with the mask. It was played for a laugh as it looks like Raiden. Later in the game, you find out that the mask just happens to look exactly like Raikov, creating the perfect disguise for Snake to get through the compound.
One of their spies becomes Raiden's girlfriend and the two fall in love. Raiden defeats Solidus after the Patriots' AIs tell him his death would also trigger those of Olga's child and Rosemary, the latter of whom is revealed to have been pregnant with Raiden's child during the mission.
Although not an eyepatch, Raiden utilized a cloth to cover his missing left eye, after a fight with Samuel Rodrigues resulted in his eye being injured and amputated, and because a cybernetic replacement eye wasn't fully created prior to his surgery, in 2018.
The now-infamous ending plot twist reveals that Venom Snake is simply a "clone" of Big Boss, a double created to distract the world while the real Big Boss betrays his allegiances to build Outer Heaven.
She's erroneously referred to as Baal — that's because the public is ignorant that the previous Electro Archon actually perished centuries ago. Ei rarely actively rules Inazuma. Her consciousness is usually holed up in the Plane of Euthymia, allowing her to dramatically extend her life.