From ROI Wiki

Tiel is the sister of Pen and one of the later batch of Burgon Lions produced during the Gorg War in Ferrel Black's northern laboratory. They are part of the third, and largest group of lions, totaling 650 in number. While Pen is the leader of this group, Tiel acts as his vice commander and advisor, and more oftentimes corrective and controlling influence, being the balancing force in Pen's life, and his leadership. The two are an inseparable pair and, even though she rags on him a lot, the two of them wouldn't have it any other way and Pen appreciates all that she does for him, even when she's yelling at him. :)