Learn English – Any term for someone who pretends to be neutral

A wolf in sheep's clothing “is used of those playing a role contrary to their real character” (per wikipedia) and may be appropriate or may overstate the case, depending on context.

An impostor is “Someone who attempts to deceive by using an assumed name or identity”; the relevant identity in this case being that of a neutral participant, a participant with no vested interest, etc.

An adjective for behavior mentioned in the question is disingenuous, meaning “Not ingenuous; not frank or open; uncandid; unworthily or meanly artful”. (Ingenuous itself has among its senses “Straightforward, candid, open, and frank.”) Duplicitous, with synonyms like deceitful, double-dealing, and two-faced, also is relevant. (Synonyms of two-faced include deceitful, hypocritical, duplicitous.) Adjective specious, “falsely appearing to be fair, just, or right: appearing to be true but actually false” also applies.

Also see: A word for a worldly wise person who pretends to be naïve?, What's single word for fake image? [with links to words like two-faced, Janus-like, deceiver, fake, double-dealer, duplicitous, polymorph, chameleon, wolf in sheep's clothing, poser, duplicitous, back-stabber, hypocrite, fraud, imposter, poseur, whited sepulcher, Devil in disguise et al], What is a word to describe a person who wears several masks?, What is an idiom/slang for "someone who pretends to be good when they're not"?, Common phrases for something that appears good but is actually bad, etc.

In some situations, the terms fifth columnist (“someone who belongs to a group that secretly undermines another organization”) and sleeper (“A spy, saboteur, or terrorist who lives unobtrusively in a community until activated by a prearranged signal; may be part of a sleeper cell”) may apply.