Among the many mysteries throughout Russell T Davies’ current Doctor Who era is a host of mysterious characters played by Susan Twist.

Twist’s first appearance was in the the Doctor Who 60th anniversary special Wild Blue Yonder where she played Sir Isaac Newton’s housekeeper Mrs Merridew on the fateful day he came up with the concept of mavity.

Her second appearance was in the Doctor Who Christmas special where she played a hippie-like lady who heckles Ruby’s band at King’s Arms with an odd request for them to play Gaudete.

Her third appearance is in Space Babies. This time we see her in a pre-recorded video as Comms Officer Gina Scalzi, protesting the abandonment of the baby farm.

Her fourth appearance is as a gobby Tea Lady at the commissary at EMI Recording Studios in The Devil’s Chord.

Her fifth appearance is as an automated ambulance interface in Boom.

Her sixth appearance is as a friendly hiker who is frightened by The Woman in 73 yards.

Her Seventh appearance is as Penny Pepper Bean, Lindy Pepper Bean’s mother in Dot and Bubble. At this point, the Doctor and Ruby both recognize her as a familiar face.

Her eighth appearance will be as The portrait in Rogue. For Twist to be on the cast list for the episode suggests that her appearance is more than a still portrait. It suggests some kind of a speaking role.

Twist appeared in one of the 2024 series trailers as Susan Triad of Triad Technology in a scene from one of the two-part finale episodes, consisting of The Legend of Ruby Sunday and Empire of Death.

Triad Technology is used by UNIT, Unified Intelligence Taskforce. The Triad Technology logo appeared on a bus used for The Church on Ruby Road filming and it also appears in one of the 2024 series trailers.

Who is Susan Triad? Fan theories vary. She has the same first name as the Doctor’s granddaughter and S Triad is an anagram of TARDIS. Her frequent appearances so far run from earthy to rude to dissenting to course to brusque and ultimately read as omnipresent, powerful and sinister. It will be interesting to see who and what she turns out to be.

The Doctor Who 2024 series continues with Boom, written by Steven Moffat and directed by Julie Anne Robinson. Boom will stream on 18 May 2024 midnight, 00:00 BST (17 May 7:00pm ET) on BBC iPlayer in the UK and Ireland and Disney+ everywhere else, where available. Boom will also broadcast on BBC One in the UK on 18 May at 6:50pm. On subsequent Saturdays, the remaining episodes in the series will stream on iPlayer and Disney+ at 00:00 BST and will broadcast in the evening on BBC One.