"The small muffled figure is none other than the dead Pacal, the father of the king-to-be, who stands facing his child in the ritual that will make him king. Chan-Bahlum designed the inner scenes of the temples to represent places in Xibalba where he would meet his father and receive the power of the kingship from him directly.
In this portal Pacal is shown giving his son the Personified Bloodletter. This was the instrument of the bloodletting rite and the vision quest. It drew the blood of the king and brought on the trance that opened the portal and brought forth the gods from the Otherworld."
Linda Schele and David Freidel, A Forest of Kings: The Untold Story of the Ancient Maya, p. 242
George Kubler notes that, in contrast to Chan-Bahlum, who appears at different ages in the three temples of the Cross Group, Pacal does not appear to age. The heavy mufflers and twisted cloth which hang down Pacal's back most likely represents burial clothing.
NOTE: What is meant by a personified bloodletter? In The Blood of Kings, Schele speaks about how ritual objects accumulated power. "In the Mayan system of imagery, this accrued power is depicted as a long-nosed personification head attached to objects. The personification head carries no specific inherent meaning beyond the concept of force. When it is attached to an object--a wristlet, an earflare or a cloth sash, for example--the personification head signals that these objects have accumulated sacred power."