Jhiaxus

Jhiaxus was the Crystal Guardian assigned to protect the Dark Spark.

Transformers: Rise of the Dark Spark
Jhiaxus was the Crystal Guardian originally assigned to protect the Dark Spark while the vault to protect it was being constructed, but started kidnapping other Crystal Guardians to test out the artifacts powers. He was caught and replaced by another Crystal Guardian. Rise of the Dark Spark Notes

Trivia

 * Jhiaxus is the only named Crystal Guardian in Rise of the Dark Spark. He is also the only Crystal Guardian with a rank as he is explicitly referred to as "Captain Jhiaxus" during the log in which he is mentioned.
 * Jhiaxus has appeared in other Transformers media as a Decepticon scientist.