Those who know Cotelin the best often see him relaxed and in an enjoyable mood. But Cotelin's demeanor varies greatly depending on the company he keeps. When he is not around those he trusts, Cotelin is guarded and serious. He holds himself as a Grand Master should, treating with others as is required by his rank. It is this same serious demeanor that Cotelin carries into combat. He understands that there is a time to be lighthearted, and a time to be serious about work. And when that work involves the death of an enemy, one does not want to be on the opposite end of Cotelin's seriousness.

Cotelin is often at the center of large developments in the Brotherhood and in his unit. Although he has largely given up on official leadership roles, Cotelin still stands tall as the leader in most of his important endeavors. In doing so, he makes tasks look easy and inconsequential. He has an ability to organize ideas, create discussion, and drive a project unlike most. And generally, the outcomes of Cotelin's projects far exceed expectations and are beyond the ability of his peers. Unfortunately, the draws on Cotelin's time are many, and these days it is a rare project that gets Jac's full attention.

Before the Exodus, Cotelin was the Emissary of the Brotherhood, charged with uniting the clans under common causes. The role was created for him because of Cotelin's unique ability to draw alliances. Jac is able to empathize with his counterparts and build consensus among groups. He has allies and friends in places that most would not expect, and it is largely for that reason that Cotelin is still alive and still respected.

There are few Grand Masters that have shown the devout loyalty that Cotelin has shown to the Brotherhood. He puts the interests of the Brotherhood beyond his own interests and is always optimistic to the future. He is just as loyal to Taldryan as he is to the Brotherhood. His home away from the Dark Council is of utmost important to him. However, the optimism of Cotelin can often be blind, and for all of his wisdom, sometimes he can be faulted for failing to account for his lack of a healthy pessimism.

Jac Cotelin has the ability to overwhelm opponents with the sheer power of the Force. However, he understands that there is a limit to his powers, and without maintaining his reserves, he may find himself lacking in a dire situation. Moreover, Cotelin understands that he is no longer at the top of his physique, though he is still strong in his own right. As a result, Cotelin will find creative ways to combine his powers and physical attacks to maximize his effectiveness.

Cotelin is the oldest member of the Dark Brotherhood, and not through mere chance. The Grand Master has lived where others have perished largely because of his approach to combat. While destruction of an enemy is key, survival is always the first goal. Cotelin survives not through cowardice and long-range attacks, but rather through slow-playing his opponents while he learns their capabilities and studies their weaknesses. Often he will allow his enemies to expend themselves at the Grand Master's strong defenses, and then take the advantage when the time is right. And when the time is right, there is no denying Cotelin's ferocity. Once he makes his move, Jac will utterly destroy those that oppose him, overwhelming them with everything he can muster.