I used ~Kuzai's Girl and Roxas base. want to use it? here's the Link: [link]

Roxas blinked his eyes open for a minute as he groaned.

"Look! the Keyblader's Nobody is waking up!" a unison of small, cute voices said as Roxas Got up, rubbing the back of his head. he looked up to see a Girl with Dark blue Hair, with one eye red and the other blue. she wore a Blue jakcet with a green shirt with long sleeves. her shoes seemed to match her shirt as he looked at the small creatures surrounding her.

"Where am I? and who are you?" Roxas said, looking at her Wings, which seem to amaze him.

She put a finger to her mouth, with a look that told him that she was thinking. she then looked at him.

"you're in the Dark Intermission. and it looks like you're trapped here just like us." the Girl said, scratching the small Puppy-like creature behind its ear.

"And who are you? Are you apart of the--"

"Organization XIII? No. I'm not a Nobody, I'm a Digimon. A Mega level to be exact. and to be more exact, My Name is Mach."

A small scene from My Fanfic, KH3: the Unlocked Worlds. the Dark Corridor is where the 'Real' Mach, her friends and the Nobodies (Including Roxas and Excluding Xemnas) are Trapped as prisoners by Malificent and replaced with Chronos Clones of Their Rookie forms (Save for Mach, who's clone just takes after her full adult form).

anyways, the reason why she's in her Youth mode is easy to explain: in the Dark Corridor, all Digimon are reduced to their Baby or In-training Levels. However, Mach's Powers and protective spells Kept her from Devolving, only reducing her to her Youth Mode, and only staying that way until she's released from the Dark Corridor, When she Digivolves into Her full Adult form.

She is the First Person Roxas Sees in the Story, she also explains to him what is about to happen to him, Namine, Demyx and Axel.

in case you don't know which are her friends are with her:
(from left to right)

Keramon: Tsukaimon's Baby Level
Mach
EclipsePaomon: Olive's Baby level
JynxYukimiBotamon: JynxGatomon's Baby level
Pabumon: (though she hasn't been introduced in the 'Final Destiny' Story yet) Junebug's Baby Level
Chibomon: Vivian's/Veemon's Baby level

Baby Digimon Sprites retrived from here: [link]

Digimon(c) Bandai/Toei Animation
Kingdom Hearts(c) Square Enix/Disney
Mach, EclipsePaomon, JynxYukimiBotamon(c)~WeBgHoSt
Base(c) ~Kuzai