Frisk (Sonic Adventure: Grounded)

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Overview
Frisk is a human who hails from the Cliffside Top Zone(?) and fell into the realm of the UNDERGROUND on their journey up Mount Ebbot. They fell into the Marble Ruins, meeting the likes of Omochao, Silver and Flicky before dueling with Silver and leaving for the Ice Cap Zone. Here, they met Sonic, Tails and Knuckles and befriended them after hijinks and the odd battle. Since then, some time passed, and on a journey through Dr. Eggman's secret lab Frisk, Sonic and Knuckles came in contact with Infinite and had their paths drastically altered by the appearance of W.D Eggman and Model C.

The House of Needlemouse: Mirror Shards
Frisk appeared in the first wave of chapters, in Chapter 3: ENTRY NUMBER 91. In the chapter, Frisk and Sonic explore Dr Eggman's secret lab and discover some of the doctor's darker sides, as well as his research into the Psuedo-Gizoid Gemerl and the Nocturnus clan. Making their way deeper into the lab, the telltale sound of a shattering mirror signified the arrival of Infinite, though they did not know it. Infinite modified their timeline by introducing W.D Gaster and placing the SOUL of Chara into Dr. Eggman's secret 'PROJECT: PRELATE'. After battles with W.D Eggman and Model C, Infinite appeared from his hiding place and had a brief, casual skirmish with the group before Sonic, Knuckles, Frisk and Model C were teleported away by Sonic's Chaos Control. Frisk's current status is unknown.

SOUL Mode
Frisk and all other residents of their universe can enter an abstraction of battle locked inside a green grid cube. While in this mode, Frisk can dodge magical attacks much easier than they can with their physical body while also, should they chose, attack using abstractions of their weapons based on timed inputs, akin to magical attacks. Frisk mainly uses this technique to pacify their opponents and end fights non-violently, but there's always the possibility of a Bad Future...

Determination
Frisk's red SOUL (and potentially the Sol Emerald encased within) grants them the ability to create a SAVE point that acts as something of a marker for time travel. Upon death, Frisk's Determination will allow them to stay strong and 'reset', bringing them back to the time of their SAVE point and creating a new branch of the timeline. Evidence from Undyne the Undying and Model C implies that Determination could allow for someone to stay alive and fighting after death or to act outside of their turn during SOUL Mode battles.

Super Form
Juddy has shown an image of dubious canonicity depicting Frisk in a Super Form facing down Solaris.

Sonic
Sonic and Frisk have been shown to be close friends, travelling together to the Lava Foundry and performing the Double Boost against Infinite. Frisk and Sonic were able to strategist during the battle with W.D Eggman right after Sonic jumped in front of Frisk to protect them.

Silver
Though he wasn't present during Frisk's journey to Eggman's lab, their relationship is noteworthy. Silver is something of a brother figure to Frisk, saving them from Omochao and carefully taking them through the Marble Ruins and allowing them to live in his house. Frisk has also flirted with them after calling him brother, which is more an account to their intense desire to flirt than anything actually serious.

Model C/Chara
Though they fought in Eggman's lab, Frisk saw Chara's potential and became desperate to SPARE them. Right before Sonic activated Chaos Control, Frisk reached out and offered their hand to Chara and took them away from the clutches of Infinite. Chara was implied to have strong feelings for Frisk, but as Chara's Frisk was from another universe, it is unknown if this particular Frisk ever returned their strong emotions.

Araki
In Sonic Adventure: Grounded's Classic Mode, Frisk teams up with Araki from Age's End, the two of them working together in a parallel to Sonic and Tails. Their relationship as close friends is noteworthy, but Araki has not so far appeared in Mirror Shards or the main RP and Classic Mode may not be canon.

Trivia

 * Due to the nature of Undertale's many timelines, it's difficult to pin down Frisk's exact alignment. The Frisk shown in Mirror Shards is left ambiguous, but confirmed not to be from a Genocide timeline (called Bad Future in Sonic Adventure: Grounded).
 * It is not apparent whether this particular Frisk made it to the Citadel at any point, as such their status is unknown.

Music

 * To be added, 'Determined To Remain (Theme of Frisk)'