Joni Shagrin was an old warhand under the command of Gareth Bryne. He used to be a Senior Bannerman of the Guards.
He was a big man with thinning gray hair.
He was escorting Siuan Sanche, Leane Sharif and Min Farshaw to Bryne's estates when he was ambushed by Logain Ablar and knocked unconscious. He set off with Gareth to track down Siuan when she broke her oath to Gareth. This led him all the way from Andor to Salidar where the Rebel Aes Sedai were.
He was sent to Ebou Dar to help recruit men for the Rebel Aes Sedai army.
Assumingly, Joni remained with the Salidar Aes Sedai all the way up to the reunification of the White Tower. Approaching the Last Battle, he started training the cavalry of the White Tower Guard together with Uno Nomesta.
After the Sharans arrived on the battlefield in Kandor and the forces of the White Tower had been pushed back to the border to Arafel, Joni was killed while commanding a contingent of the White Tower Guard fighting for control over a river ford. He was mourned by his lord, who however stated that Joni had always wanted to die in battle.
Uno replaced him when he died.