Mostrum Appatine is a newly-qualified counterpart who started his probation under the tutelage of Shard Ider Firenz. They were given a routine diplomatic close protection assignment on Fengrir for his first active duty mission (→SFTD).
Appatine was seriously injured during a fight with ex-counterpart Maber Castigon. He was disabled by the insertion of a knife blade between the vertebrae of his spine, and was left unable to walk. Appatine's failure to stop Castigon allowed the renegade to kill Firenz (→SFTD).
After being retrieved from Fengrir, Appatine underwent emergency surgery to fix the damage done by Castigon. At the current time he has had contact plates fitted for a neural bypass in his spine, and is awaiting the fitting of a shunt which will bridge the damaged nerves and allow him to walk (→TRD).
Despite not being fully recovered, Appatine contacted Elm Caden in the hopes that he might be chosen as a replacement counterpart in the absence of Rendir Throam. His intention was to assist Caden in hunting down Castigon. Due to Appatine's injuries, and his relative inexperience, he was turned down by Caden. Caden was however impressed with Appatine's determination to locate and deal with Castigon, despite the unfortunate consequences of their last encounter (→TRD).
Mostrum Appatine may have been knocked out of action early on in GWOTS, but as some readers have guessed his story does not end there. His enthusiasm for getting back into the fight — despite a serious injury — is very much a core part of his personality.
Readers will have to wait to see if he has any role to play in the final showdown with Voice and the Shaeld Hratha, and whether he will survive to take part in any future series.