Tarah is somewhat of a mystery in Kaladin's past. It is assumed that she too is/was an Alethi darkeyes. She and Kaladin were obviously close in some way. Whether such closeness was romantic or platonic remains to be discovered.
Tarah coaxed Kaladin out of his single-minded determination in training with the spear due to his guilt over Tien's death. She claimed that it would burn him out.
Kaladin has referred to her some few times in The Way of Kings and in Words of Radiance.
When Adolin speaks with Kaladin about Shallan, Kaladin's advice is for the heir to be frank with Shallan about his feelings. He then thinks about Tarah and the mistakes he made with her.
Later when Kaladin and Shallan are in the chasms and they are huddling together for warmth, he realises that this is the closest he has been physically to any woman since Tarah.
" ... I let Tien die, I failed my spearmen, the slaves I tried to rescue, Tarah... ." He hadn't thought of her in some time. His failure with her had been different from the others, but was a failure nonetheless.
What would have happened to him, if Tarah hadn't coaxed him out of his single-minded dedication? Would he have burned himself out, as she'd claimed.