Aristeides was a female Changed, a type of humanoid who was genetically engineered to adapt to life on high-gravity planets, with thick bones and layers of muscles. She was also immune to human diseases and plagues, and severe trauma is the only way that she could be injured.
With these traits in mind, Aristeides joined the Enterprise's security division, under the command of Lieutenant Commander Mandala Flynn. However, she did not fit well in security, preferring to work in the ship's botany section. As a result, Commander Spock had Aristeides transferred to the botany section. (TOS novel: The Entropy Effect)