This is another concept where I'm not sure it would actually make a great character for a new player; however, I had the idea and statted it up, so I'm going to post it now.
For when your Diedne infiltration takes an unfortunate turn: a Cuckoo Bjornaer.
Characteristics: Int +1, Per +2, Pre +2 Com +2 Str +0, Sta +2, Dex -4, Qik +0
Size: 0
Age: 25
Decrepitude: 0
Warping Score: 0 (0)
Confidence Score: 1 (3)
Virtues and Flaws: The Gift; Hermetic Magus; Diedne Magic, Ghostly Warder; Heartbeast* (cuckoo), Protection, Quiet Magic x 2, Subtle Magic; Dark Secret, Greedy, Twilight Prone; Incompatible Arts (MuAn and ReMe), Infamous, Obese, Poor Dexterity
Personality Traits: Gluttonous +6, Lazy +3, Manipulative +2
Reputations: "Probably an imposter of some kind"
Combat:
Dodging:
Soak: 0
Fatigue Penalties: OK, 0, -1, -3, -5, Unconscious
Wound Penalties: -1 (1-5), -3 (6-10), -5 (11-15), Incapacitated (16-20)
Abilities: Artes Liberales 1 (spontaneous magic), Awareness 2 (watchers), Brawl 1 (dodge), Charm 2 (intended patsies), Code of Hermes 1 (Diedne), Concentration 1 (spells), Finesse 2 (Imaginem), Folk Ken 2 (magi), Guile 3 (lying to authority), Intrigue 2 (magi), Latin 4 (Hermetic terms), Legerdemain 2 (filching), Magic Theory 3 (Imaginem), Native Language 5 (storytelling), Order of Hermes Lore 1 (Diedne), Parma Magica 1 (Mentem), Stealth 1 (rural areas)
Arts: Cr 0, In 0, Mu 5, Pe 5, Re 0, An 0, Aq 0, Au 0, Co 0, He 0 Ig 0, Im 9, Me 0, Te 0, Vi 9
Twilight scars: None
Equipment: Wizardly robes
Encumbrance: 0 (0)
Spells known:
Winter's Icy Touch (PeIg 10) (Mastery 1: Multiple Casting) +8
Aura of Enobled Presence (MuIm 10) +16
Disguise of the Transformed Image (MuIm 15) +16
Image Phantom (MuIm 20) +16
Veil of Invisibility (PeIm 20) +16
Unseen Arm (ReTe 5) +2
Wizard's Vigil (MuVi 20) +16
Masking the Odour of Magic (PeVi 20) +16
Character Notes
When this character was but a young apprentice a ghost came to them and told them their it was their ancestor, a great wizard in their own right. They offered to guide them to secret magics, and the child eagerly agreed.
Some years later, the child went through the Ritual of Twelve Years, which revealed their Heartbeast to be a Cuckoo. This immediately caused suspicion from the assembled Bjornaer, some of whom accused the child of being an infiltrator from an enemy cult of shapeshifters, but they were protected by their master - a well respected maga who could not believe ill of their apprentice. Still, some precautions were taken, and to this day certain kinds of magic are impossible to the character.
Customisation Notes
This character's infiltration could be made more subtle by changing the Heartbeast - a snake would keep the general theme of deception whilst having more respectability within the House, or an alternative Heartbeast could focus on other aspects of the character. If changing the Heartbeast you will probably also want to change the Personality traits and flaw.
Alternatively, "Dark Secret" could be swapped out for "Plagued by Supernatural Entity", and Ghostly Warder swapped for other virtues.
Other Notes
This is stretching what "Ghostly Warder" is really meant to do.
If using out-of-core books, I'd be tempted to give the character the "Slothful" personality flaw, or the "Persona" virtue.