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:
*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.