Companion Rib is obtained by reaching a high level of trust with Skelly, and then gifting him Ambrosia.
Using Rib in battle will summon Skelly to aid you. He will appear and distract foes in an area by shouting at them, drawing their attacks and attention away from you. He can take friendly fire. He will remain until he is killed; if you finish the encounter before he's killed, he'll still be standing in the empty chamber when you leave.
Rib cannot be used outside of combat encounters or while fighting Charon.
Like other Companions, Rib can only be used once per combat encounter, and up to 5 times per run (upgradable with Ambrosia) or more if the Night Spindle is bought from the Well of Charon.
'...When Rib first emerged from the darkness, he wished more than anything to be as strong as many great heroes who reside within the Underworld. But he was distraught by his thin frame, and found there was nothing he could do to change it, no matter how he tried. However, Mother Night cautioned him that he was following a path to greatness very different from the heroes he admired; they were guided not by envy of others. Rib did not believe this; he knew heroes inspired other heroes. Determined to prove himself, he sought to hoist an entire boulder to prove his strength to those he knew. He trained and trained. But by the time Mother Night found him, he was crumpled in a heap. Now, Rib no longer seeks to be admired, but is very capable of making himself useful to whomever he is given to. And, he is not so preoccupied with heroes, anymore.'
Once Zagreus forges a bond with Skelly, the text becomes "From Skelly, willing to die for you"