The class is reminiscent of a certain movie you guys know well with the black parasite or a pink one from a favorite anime series. The idea of its abilities mechanically is that the player would attach to a host whether enemy or ally for a certain length of time ( if an enemy then the player would control it like normal but with limited abilities and it has to be organic; if a player then you will use abilties on or around the player to support the team). It will be unique from most of the classes in the game but also it will play as a support class.
So I would imagine, the player whos getting bonded to will get a prompt to allow it to latch on, then when the other player gets on the host, the host will still be in control but with extra features and abilities. When the host is defeated, the Symbiont takes over for a brief period of time before it detaches and becomes blob form again, lets go with 2 minutes.
While in blob form, it has the ability to shapeshift into any "organic" enemy. So mechanical enemies don't count. I am not sure about supernatural enemies like ghosts or demons as such, maybe they were organic so in some sense it might be possible to shapeshift. While shapeshifted there is limited abilities as well because blobs can't use magic or shoot beams of light and such.
That's my idea of the class. I had to get it out so i won't think about it.