All credit for Warframe abilities goes to my best friend.
STORY
"A Warframe corrupted by the Void, its Operator still searches for their lost partner to this day, even if it means becoming corrupted themselves."
Kevalin - the lost Warframe's Operator - is a Quest-specific character that would be introduced in the "Corruption Of Mondar" questline. In his desperation, he reaches out to the Tenno to assist him in searching for his old companion, as his exposure to the raw Void begins to corrupt him too.
The Quest would involve following up on clues about Mondar's whereabouts, learning more about Kevalin's dedication to this specific Warframe, and trying to find a cure for the two's corruption before it spirals out of control.
Upon completion of the Corruption of Mondar quest, players would be able to select Kevalin as a playable variant of the Operator in the Transference room, as well as gain progress towards unlocking Mondar himself as a playable Warframe.
GAMEPLAY - KEVALIN, "EDGE OF CORRUPTION"
As an Operator corrupted by the Void, Kevalin's powers differ from those of the Tenno:
Rapid Combat - When using Transference, Kevalin dashes forward from the Warframe for a short distance. If he impacts with a surface during this dash, Kevalin causes an explosion of Void spikes around himself, dealing damage to nearby enemies.
Tainted Shield - Using the Void Mode input causes Kevalin to create an impenatrable shield of Void spikes around himself. Every time the shield takes damage, it consumes a small amount of energy, and will disappear when the energy runs out.
Tainted Step - Using the Void Sling input causes Kevalin to blink in the targeted direction. Upon exiting the blink, nearby enemies receive a small amount of Puncture damage, and are slightly slowed down for 2 seconds. Holding the input gradually increases Tainted Step's range, while also allowing better aiming for your desired location.
Tainted Spike - Using the Void Beam input causes Kevalin to launch out a set of Void spikes towards the targeted location. Unlike the Tenno's Void Beam, Kevalin's primary attack has a slow rate of fire, but grants greatly increased damage in return. Because of his corruption, Kevalin is unable to make use of AMPs.
Pure Release - Using the Void Blast input causes Kevalin to release a blast of pure Void energy in a small area in front of himself. Holding down the input allows for continuous damage, at the cost of rapidly draining his energy. Pure Release can still be used when energy is depleted, but will begin to cost 1 health per second to use.
GAMEPLAY - MONDAR
Passive - Negative Status Effects affect Mondar 50% less (Reduced slow from cold, lower damage from bleed, faster recovery from knockdown, etc.).
Ability 1 - Summon the Void Angels' claws to burst from the ground. Press the ability button again while the claws are active to cause them to rapidly close in on the center. Hold down the ability button in either state to get a better indication of the claws' damage area.
Ability 2 - Transfigure your corrupted body, and wreak havoc with an exalted Void shotgun from Mondar's arms. Headshot kills have a chance to cause the enemy to explode, dealing Void damage in a small radius.
Ability 3 - Sacrifice a small protion of health to cover yourself in tainted Void spikes. Upon activation, incoming damage is absorbed, and once the invulnerability phase ends, any incoming damage is reflected back at the attacker, with the previously absorbed damage being added on top of the enemy's dealt damage.
Ability 4 - Mondar unleashes the Angels' Song, trapping nearby enemies in an immobilized state. Enemies affected by the Song will have their headshot hitbox increased, and headshot kills from any means will restore the attacker's health.
DETAILS AND REFERENCES
The name Kevalin is a name of Sanskrit origin, and means "Seeker of the absolute." As his character's entire purpose is to seek out his old partner and Warframe, i felt like the name was appropriate.
The name Mondar is an altered version of Moander, the god of rot, corruption and decay from Forgotten Realms. I was hoping to use a real god or deity for his namesake, but ended up going with this after i couldn't find anything
Quick design reference for Kevalin - Transference suit overflowing with Void energy, weak and uneven stance
Close up of Kevalin's face - Uneven eyes whose color is lost to the corruption, Void contamination covering his face like veins
TL;DR - Corruption of Mondar quest introduces Kevalin, the Operator fighting Void contamination, and Mondar, his fully corrupted Warframe companion. It involves tracking down Mondar, while also trying to find a cure for the Void corruption plaguing the two.
Kevalin is a playable variant of the Operator (Like the Drifter), but with skills more focused around Void corruption and cannot equip AMPs.
Mondar's abilities make use of his Void-stricken body. He summons claws of the Void Angels, morphs his body into an Exalted Shotgun, and summons a Void Entity whose effects change depending on the active abilities active before casting.