Yellow

Red, Blue, and Green were the original Pokedex holders - their efforts saved the region of Kanto from the devious intentions of Team Rocket. but there is a fourth dex holder - Yellow. in the aftermath of Rocket's take-down, Red went messing. and finding Pika, his Pikachu, Yellow took it upon herself to uncover what happened to him.

but there's much more to Yellow than meets the eye, even if she herself is unaware of the true scope of who and what she is. humans don't have an innate ability to talk to pokemon, after all. nor do they have the ability to heal wounds with a touch. some perhaps have psychic abilities, which have barely been researched as it is - but Yellow is an outlier even then, an rare phenomenon that seems connected to the Viridian Forest. but... the details of that are known even less - it might as well be a child's fairytale.

what is a Child of Viridian, exactly?

TL;DR: Yellow is quite naïve and acts very much like a little child. Despite her childlike nature, she can be very serious when people and places she cares about are threatened.

TL;DR: She hates it when Pokémon are injured, hers or her opponent's, and has developed a style of battle in which neither her nor her opponent's Pokémon are hurt unless it is absolutely necessary.

TL;DR: 

She received psychic abilities from Viridian Forest, which include the ability to read the emotions of Pokémon and heal their wounds.

Limited telekinesis, enough to manipulate a Poké Ball on a string.

The ability to read the minds, emotions, and memories of Pokémon.

Healing abilities - both humans and pokemon.

Able to "synchronize spirits" with her Pokémon to boost their levels into the mid 80s in times of need.

9 (RGB) / 11 (Y) / 12 (GSC) / 14 (FRLG) / 15 (E)

~ EST. 18 (HGSS) / 20 (DP) / 23 (BW) / 25 (B2W2) / ?? (XY) / ?? (SM) / ?? (SwSh)

Tl;dr: two years younger than Red