Light Yagami/Harry Potter
Light moved his hands and feet, trying to measure what range of movement was available to him. Then he flexed his toes and clenched his fists to gain a measure of his strength.
Light wondered if his infant body will also effect his intellect, same as it was effecting his attention span, language of thoughts and lack of emotional control. Does intelligence come from one's body or soul? The realist part of his brain, scientifically inclined, said it's his body. The other part, one who has seen Death Notes, Shinigami, and apparently transmigration- or is it reincarnation?- said it's his soul.
The scared part of his brain concludes that it is a mix of both. His soul was that of Light Yagami, certified genius. But his body was that of an infant. And if his infant's body does not develop the same brain capacity for higher thought that his previous body did, then he might as well be a pauper.
Under the cacophony of sobs and reassurances in the background, Light's mind started going over basic math, from quadratic equations to trigonometric proofs. He needed to train his mind back to his peak.
Not a single minute can be wasted.