Greymon is Kiriha Aonuma's partner Digimon and a member of his Blue Flare team. 

Taiki Kudou, an upbeat seventh-grade boy who always helps people in need, encounters a Digimon named Shoutmon, who brings him and his friends to the Digital World. With his friends Akari Hinomoto and Zenjirou Tsurugi, they form Team Xros Heart to compete against rival teams battling for supremacy, with their ultimate goal of taking down the evil Bagura Empire, who plans to conquer the Digital World.
 
(Source: Anime News Network)

Returning to the Digital World, Taiki and Shoutmon learn that the Bagra Army has taken it over and reformatted the world to have Seven Kingdoms, each ruled by a general. 
(Source: AniDB)

It starts off with Taiki having a dream about leading a Digimon army with himself as its "General" against one consisting of several Mammon, Rhinomon and Valvemon. He then Digixroses Shoutmon, Balistamon and Dorulumon into Shoutmon x4 who effortlessly takes out most of the oppossing army. Oversleeping, Akari wakes him up with a "move" she calls "Akari Strike". 
After being encountered by Zenjirou, Taiki suddenly hears a melody from out of nowhere with Shoutmon crying out. Taiki starts searching for the source of the voice in order to save him. Chasing after him, Akari and Zenjirou witness a car about to fall on top of Taiki, but he narrowly escapes its path thanks to Shoutmon's warning. Suddenly, the buildings and surrounding space begins to distort, causing electronic disturbances. 
Afterward, Taiki discovers a light along with a melody, then a mysterious voice says "do you want to save this melody?" and a Xros Loader appears. After following further instructions from the same voice, and once the melody is absorbed by the Xros Loader, Taiki, Akari and Zenjirou fall into a wormhole-like digital space which emerges on the ground, transporting them to the Digital World. Once there, they are in the middle of the Bagura Army, the same army Taiki saw in his dream, lead by Mach Leomon.

Children nowadays are enthusiastic about hacking on their flip phones and Digimon. Hinata gets her long-awaited "flip phone''...
 
Note: Honorable Mention in the 2023 Digimon Comic Awards.