Allie Jamison & Dr. Barrister
Allie Jamison and Dr. Barrister were both introduced as characters by Mary Carey in The Mystery of the Singing Serpent. Allie had an unwelcome houseguest that she wanted the boys to help evict, and Dr. Barrister, posing as Bentley, was the houseman for Allie's home.
Allie was eventually proven right when it turned out that her houseguest and her aunt were involved in pseudo-demonic activity. Dr. Barrister was exposed by the boys as someone other than a houseman; however, it wasn't until the end of the story that his role as a researcher was brought to light.
Both characters make one more appearance in a T3I story, albeit different stories. Carey brought Barrister back first in The Mystery of the Invisible Dog, where he was used to help steer the boys to a fellow researcher who had experience in astral wanderers. Allie Jamison was revived by Carey in The Mystery of Death Trap Mine where she again served a major role by blackmailing the boys to go to New Mexico to her uncle's Christmas tree farm to help solve a mystery.
Illustrator Ed Vebell created the original drawings for both characters in The Mystery of the Singing Serpent, and he told me that he modeled Allie after one of his daughters and he modeled Barrister/Bentley after himself.