Taniguchi.
Summary from Seiji:
>Lancelot's Noble Phantasm (unique skill), For Someone's Glory, allows him to disguise himself as (effectively take on the aspect of) other Heroic Spirits. Lancelot summoned as a Berserker (in the case of his Fate/Zero counterpart) has a degraded version of this Noble Phantasm due to his Mad Enhancement (it only manifests itself as a black mist that surrounds Berserker and blocks attempts at divinating his identity and status), and the true version can only be invoked via a Command Spell (the seals on the Master's hand), albeit the disguised aspect still appears to be under Mad Enhancement.

>In this short comic, Lancelot's Master (Fate/Grand Order protagonist, nicknamed "Gudao" and afterwards given the name Ritsuka Fujimaru in the anime adaptation) uses a Command Spell to order Lancelot to take on the form of servant Tamamo no Mae. He then proceeds to, uh, do what he did. Albeit to be honest, if the comic were more accurate Gudao would definitely need to use an extra Command Spell to keep Lancelot from brutally murdering him before he even gets to do anything with him (a Berserker is usually an extremely unstable servant and is only moderately loyal to his Master, plus even if driven insane by the summoning ritual they do not completely lose awareness).

Btw I used to play Fate GO and I rolled and got Tatamae no Mae. But it got boring grinding so I sold my account to a man who wanted her for about $70 without putting a dollar into the game, just like a chad.