Angemon's Sacrifice
The last partner to reach Champion in Adventure and the anime's first partner death: Angemon spent all his power to defeat Devimon, reverting to a Digi-Egg and restarting the cycle from Poyomon.
Every Devimon Is a Fallen Angemon
Devimon's official profile says it was an Angemon that fell into the Dark Area, with the Mark of Evil on its chest as proof.
Tsukaimon, the Familiar
Named after the tsukaima, the occult familiar spirit: it looks like Patamon but serves Devimon. In Adventure: 2020 a Tsukaimon starts the Devimon line.
Daemon, the Fallen Seraphimon
Seraphimon's official profile states that Daemon is said to have originally been a Seraphimon that fell to the Dark Area after rebelling against the embodiment of good.
Daemon (Mantled)
Under this crimson mantle Daemon led the Daemon Corps in Adventure 02; too powerful to defeat, it was banished into the Dark Ocean. A look, not an evolution.
ShadowSeraphimon
The Frontier dub name for BlackSeraphimon: officially a form of Seraphimon engulfed by its concealed inner resentment. In Frontier it is Mercurymon fused with Seraphimon's stolen data.
One Absorption from the Top
Daemon (Ultra) exists only in the V-Tamer 01 manga, where Daemon absorbs Arcadiamon's data and surpasses the Mega level.