Rupert???? The silver band is his ring for sure. Tatiana's insanity started from the demon pox attack. It is common for people who underwent trauma to try to see how things could have turned out different after overanalyzing their traumatic experience. Maybe, Rupert's strength got stronger due to the manner of enchantment placed on the dagger rather than it's relations. He may also have known Elias well. Feel free to add your thoughts for this ramble while I do my sehri in preparation for my Roza.
Happy Ramadan
Or it's James. He was extremely concerned with Matthew's drinking, the annoyed shake may be him being annoyed at them for dredging up memories of his parabatai or may even be him sobbing or dry-heaving at Matthew's name, he was bound to Grace since childhood, Lucie risked herself for Jesse Blackthorn, he could easily recognise Alastair's dagger and to Cordelia and Alastair, Cirenworth was the home of the Carstairs. Also the wine flask may remind him of what Cordelia suffered due to Elias. He is also mentioned in the Bane Chronicles, The tales of the Shadowhunter Academy and in the epilogue of the TID series. His ring (a silver band with herons) was also given to Jace and Jace gave it to Kit. Also he was mentioned by Tessa and Clary in the epilogue of a City of Heavenly Fire.
GUYS IT COULD BE RISA OR SONA
DISLIKES MATTHEW ( Risa does, Sona could after Cordelia ran away with him) ✅
Has something to do with Alastair ✅
I think it's Sona Carstairs, nee Jahanshah. Cordelia was almost killed due to Tatiana, Alastair has his heart broken due to Grace and so did Cordelia, not only that but the flask was clearly a wine flask and she may have had an adverse reaction to it due to Elias. Also at her depths Sona always considered herself a Jahanshah.
GUYS IT COULD BE RISA OR SONA
DISLIKES MATTHEW ( Risa does, Sona could after Cordelia ran away with him) ✅
Has something to do with Alastair ✅