|Beverley Cressman as Kate Lethbridge-Stewart|
I'm sure it's no coincidence that the character leading the Big Finish charge into the realm of the new Doctor Who is Kate Stewart, who, herself, began life in spin-off media.
For those not up to speed on the Brigadier's daughter, check out the Wikipedia information on her from its UNIT Personnel entry:
Kate Stewart or Kate Lethbridge-Stewart (her birth name is mentioned by the Doctor in The Power of Three, 2012, and he addresses her as such in The Day of the Doctor, 2013, and credited as such in Death in Heaven, 2014, and is called "Tiger" by her father) originated in spin-off media. Kate is introduced in the home video release, Downtime (1995), in which she is the estranged daughter of retired Brigadier Lethbridge-Stewart and single mother of the young Gordon Lethbridge-Stewart; the family reunited at the end of that film.
She takes a more active role in defending the Earth from alien forces in the sequel, Dæmos Rising (2004), in which she starred.
Kate first appeared on Doctor Who in the 2012 Eleventh Doctor story The Power of Three as UNIT's Head of Scientific Research. She had dropped Lethbridge when joining UNIT so as to avoid the appearance of nepotism and to be judged on her own merits.
Nevertheless, her father had mentored her until his death in early 2011, and always taught her that "science leads" - a motto he had attributed to the Doctor. This motto eventually culminates with her directorship of UNIT and the radio call-sign "Greyhound Leader", previously held by her father, Brigadier Winifred Bambera, and Lockwood.
Kate is portrayed by Beverley Cressman in Downtime and Dæmos Rising, and by Jemma Redgrave in The Power of Three, The Day of the Doctor and Death in Heaven. Each actress also portrayed an alien who disguises itself as Kate.[note 3]
Kate Stewart will return in two separate two-part stories in series nine. Firstly in The Magician's Apprentice/The Witch's Familiar and then in a later story featuring the return of the Zygons.
|Jemma Redgrave as Kate Lethbridge-Stewart|
The TARDIS Data Core has a more in-depth examination of Kate Stewart's 'in-universe' backstory, but points out that "Jemma Redgrave, who is naturally brunette, plays the role of Kate with blonde hair matching Beverley Cressman's portrayal of the character."