Romana is a Time Lord who accompanied the Fourth Doctor on many of his adventures.
Is Romana still the only Time Lord to be one of the Doctor's companions?
For the purposes of this question, I would set the bar for companion status as follows:
- Must be a friend of the Doctor, even if there was a disagreement, divided loyalties (e.g Turlough), or they parted on bad terms
- Must have travelled in the TARDIS along with the Doctor
- Must have appeared in more than one episode if this is a TV series companion
- Must not be the Doctor (as in multi-Doctor episodes)
- Must not be the Master, even in a sympathetic guise*
EDIT: I will upvote high-quality answers where Time Lord status is debatable, but only accept answers where Time Lord status meets the following strong criteria:
- Must be Gallifreyan
- Must have Time Lord DNA
- Must have been a Time Lord since childhood
FINAL EDIT:
*I will upvote, but not accept, a high-quality answer that explains why the Master (as Missy) counts as a companion. Thank you to those posters who pointed out that there was a reasonable argument for this.