Below are the mentioned characters that specifically hail from the Ben 10 franchise. Incidentally, they all pertain to Ben's alien transformations. Since Ultimate forms did not yet exist as of the Birthday Bash, items and nanos related to them are not available in Retro.
AmpFibianSee Also: AmpFibian on the Ben 10 Wiki
AmpFibian is is an electric jellyfish-like species known as an Amperi from the planet Tesslos, which is in the andromeda galaxy. Like with Ben's other alien forms, Lord Fuse wants to create a fusion copy of AmpFibian using the replicative powers of Echo Echo. His Fusion Echo Echos steal the AmpFibian data from Dexlabs to accomplish this, and Fuse successfully creates a Fusion AmpFibian. Defeating it results in a Nano. The AmpFibian Nano can be seen in the Dexlab's Nano Research Facility being worked on. It is notably a form of the Unstable Nano. AmpFibian also appears as a full collectible costume set that the player can obtain.
Big ChillSee Also: Big Chill on the Ben 10 Wiki
Big Chill is a Necrofriggian, natively from the Kylmyys. He is an ice alien with the ability to fly and become intangible. In Ben 10: Ultimate Alien he was given an ultimate form, which granted him the ability to breathe ice flames.
Big Chill and Ultimate Big Chill themed costume pieces are also available in the game.
ChromastoneSee Also: Chromastone on the Ben 10 Wiki
Chromastone is a Crystalsapien from the planet Petropia. He is a crystal-like alien with energy absorption abilities.
Chromastone was one of the aliens Fuse attempted to make a fusion of, having stolen one of its crystals. The player ends up recovering Chromastone's samples, however, and brings them to Dexter for nano development.
Chromastone also serves as the basis for a hat design.Vilgax's Ship as a trapped alien that the player must save.
See Also: Ectonurites on the Ben 10 Wiki
Ectonurites are an alien race from the planet Anur Phaetos, and the subject of various guide missions handed out by Ben. They are a ghost-like species whose abilities fall in line with traditional ghosts—invisiblity, intangibility, possession. Several NPCs state that Econurites are one of the few known species whom repelled the Planet Fusion invasion on their world. In order to save Anur Phaetos, they were forced to assist Lord Fuse in his subsequent invasions, which only Vilgax seems to know the details of. The player can find an Ectonurite ship crashed in the Nowhere Triangle, emitting a scrambling signal that prevents Grandpa Max from launching his satellite.
GhostfreakSee Also: Ghostfreak on the Ben 10 Wiki
Ghostfreak is an Ectonurite who was originally meant to be a nano, but was replaced due to the release of Ben 10: Alien Force. Ghostfreak remains depicted on rides within Dizzy World. There are also items with Ghostfreak-inspired designs.
HumungousaurHumungousaur on the Ben 10 Wiki
Humungousaur is a Vaxsaurian from the planet Terradino. He is a strong, humanoid dinosaur alien with the ability to grow in size at will. Combined with an Ultimate form, he is one of Ben's most physically powerful alien forms. Like Ben's other aliens, Fuse stole a scale from Humungousaur with the intention of making fusion copies of him. The player can get his nano upon defeating Fusion Tetrax. Humungousaur is the final nano of the game, at level 36 (32 post-Academy). Humungousaur and Ultimate Humungousaur are also hat designs the players could collect.
See Also: Jetray on the Ben 10 Wiki
Jetray is an Aerophibian from the planet Aeropela; an aquatic, aerobatic, manta ray-like alien with the ability to reach high enough speeds to enter hyperspace. When Ben could last turn into Jetray, one of his talons were stolen by Fusion Gwen. Fearing potential Fusions of his aliens, Ben has the player retrieve it in hopes that Dexter can use it to create a nano of Jetray instead. Additonally, Jetray's wings are a wearable item.
RathSee Also: Rath on the Ben 10 Wiki
Rath is a tiger-like alien known as an Appoplexian from the planet Appoplexia. Rath is a physical powerhouse that makes Ben more aggressive, often using the phrase "Let Me Tell Ya Something!" Upon investigating Mojo's Volcano, the sightings and incidents of of Rath causing trouble are revealed to be another Appoplexian that Albedo had captured. Defeating it earns the player a Rath nano. 
SpidermonkeySee Also: Spidermonkey on the Ben 10 Wiki
Spidermonkey is a blue, four-armed monkey alien known as an Arachnichimp, who also has an Ultimate form. Like Ben's other aliens, Spidermonkey is a subject of interest for both Fuse for Fusion creation, and Dexter for Nano creation. The player tracks down Fusions Gwen and Ben to retrieve Spidermonkey's fur and prevent them from creating such Fusions. Although one is stated to be in development, there is no Spidermonkey Nano in the actual game.
Spidermonkey also makes up an obtainable costume set.Ultimate Spidermonkey's spider legs
SwampfireSee Also: Swampfire on the Ben 10 Wiki
Swampfire is a Methanosian, a fire and plant based species from the planet Methanos. Along with the standard Swampfire, the evolved form "UItimate Swampfire" was introduced in Ben 10: Ultimate Alien.
Swampfire, along with Ben's other alien forms, are the subject of a project by Fuse to create Fusion counterparts of them. Ben has difficulty getting the Omnitrix to work due to interferences; whenever it does function, Fusion Gwen shows up and takes a piece of the alien. The player ends up tracking down it and Fusion Ben in order to stop Fuse's plan. The swamp root taken from Swampfire helps form a Nano of him.
Both the regular and Ultimate versions of Swampfire appear in costume pieces.
UpgradeSee Also: Upgrade on the Ben 10 Wiki
Upgrade is a Galvanic Mechamorph from the moon Galvan B. He is a technokinetic alien that can enhance any piece of technology he merges with. 10-year-old Ben uses Upgrade to bypass Mojo's Volcano's security system. As with Ghostfreak, Upgrade was intented to be a nano until the game shifted to using Ben 10: Alien Force's cast. Nonetheless, Upgrade still makes an appearance in the game as the deisgn of obtainable clothing items.
Way BigSee Also: Way Big on the Ben 10 Wiki
Way Big is ginormous creature from a species known as To'kustars, who are all born in cosmic storms. In Retro, Way Big is solely represented through a costume set.Ice King's Palace, and an unused Nano well.