She's a little easily excitable and loves space almost as much as Yuki does. She's also taken with Gentaro, as it is a combination of jealously to Yuki over how close she is to Gentaro and how strong her desire is to go to space that she's made it clear she wants to be the best one in the astronaut exams. She is also cold and calculating, and takes no shenanigans with the exams.
Due to her history of being raised in the United States, Erin's Japanese is far from perfect, and she mixes up words that sound somewhat similar, such as "flawless" and "filthy".
Erin Suda was transferred to AGHS from an American school to pursue her dreams of becoming an astronaut, because her father was one and she looked up to him, even though he retired before ever going into space. She seeks to be the first in her family to go into space, and does her best to be even better at the astronaut tests than Yuki. She wants to be an astronaut because her father was one also, but he was never able to go into space because he was outdone by someone who had less skill, but was more charismatic.
She takes up the Aquarius Switch because of the advantages a Zodiarts body will give her in space later on in her career, but vows to become an astronaut on her own skill. Erin accidentally reveals herself to Gentaro because of her shoddy Japanese. So he strikes a deal with her; if Yuki aces the Astronaut Selection Exams, Erin will give Gentaro her Zodiarts Switch.
However, everything went wrong after the second exam, in which the two teams were tasked with building a robot from a provided kit. The team ended up choosing Yuki's design over Erin's, and when Erin went back to the testing room and found a missing piece. In her blind anger, Erin became convinced that Yuki hid the piece on purpose in order to get her own design chosen.
During the third exam, Erin snuck away from her partner Kengo and attacked Yuki as Aquarius, causing Yuki to sprain her ankle. However, Gentaro intervened, explaining to Erin that Yuki hid the piece because she was told to; each box had a note, instructing the reader to hide a piece in order to test how the team dealt with the unexpected. Kengo corroborated this, saying that he found the note in his own team's box and likewise had to hide a part. Realizing her mistake, Erin returned to Yuki, healing her ankle and apologizing for her misplaced anger, saying that if she did win she wanted to do so fair and square.
After the exam, Erin rejected Gentaro's offer of friendship out of loyalty to Gamo; respecting her wishes, Gentaro challenged her to one final battle. During the fight, Fourze managed to destroy Aquarius' her healing jars with a Claw Switch-powered Barizun Sword slash and took the opportunity to defeat her with the Rider Super Galaxy Finish. Virgo used her powers of illusion to sneak Erin away, recovering her Horoscopes Switch for Gamo and allowing them to erase her memories. She met Gentaro and Yuki at the site of their original meeting, the JAXA building, and she eagerly became friends with both of them.
As the Aquarius Zodiarts, she is armed with the Nectar multi-headed whip, which can stretch to great lengths.
This Zodiarts also has the ability to emit water from the jar-like shoulder pads, which has the power to heal any inflicted wounds or bouts of sickness. When in battle, they automatically heal any injury the Aquarius Zodiarts has sustained. This healing ability is strong enough to repair damage from a Rider Super Galaxy Finish from Fourze's Cosmic States. If one of the shoulder pads is destroyed, then the other would heal the damaged shoulder. The only way to negate the healing ability the Aquarius Zodiarts has is to destroy both shoulder pads at once, as Fourze did with the Claw Switch-powered slash from the Barizun Sword.
Due to the short amount of time, Erin had never managed to truly master her own Switch, as Tatsugami has shown the Nectar has the ability to be used as a tracker.
Erin's name can be anagramed to "Eridanus", which is a river in Greek mythology that has ties with Aquarius the Water Bearer in some versions of the myth.