Nothing is known of Kapena's early life except that he was born into a family from the lesser strata of Hawaiian nobility, subordinate to the high chiefs or aliʻi nui.

His classmates included historian David Malo and Samuel Kamakau and politicians Boaz Mahune and Timothy Haʻalilio.

Kapena became the secretary and advisor to Kīnaʻu, the Kuhina Nui (an office similar to that of a prime minister or co-regent), and represented her in the drafting of Hawaii's first constitution and declaration of rights.

In an effort to establish a stable government against colonial ambitions, King Kamehameha III, the Kuhina Nui and the council of chiefs sought to make Hawaii a constitutional monarchy.

Kapena and Boaz Mahune (representing the King) assisted American missionary William Richards in the endeavor of drafting this document.

[2] Mahune and the graduates at Lahainaluna were chiefly credited with drafting the Declaration of Rights of 1839 in the contemporary newspaper The Polynesian.

However, research by Hawaiian historian Jon Kamakawiwoʻole Osorio credits Richards as the actual author of the Declaration and the majority of the later 1840 Constitution while Mahune and Kapena were only assistants.

[3] The 1840 constitution codified the existing political structure of the kingdom and created the Hawaiian Supreme Court and the Legislature of Hawaii.

In order to replace the diminishing number of aliʻi nui, it was decided by the existing members of the House of Nobles on April 2, 1845 to vote into the council lower ranking chiefs who were "men of learning" and elevate their chiefly statuses.

Kapena led the procession at the funeral of all six of her family members including her two brothers, nephew, cousin and father.