This is a thread and post I found on the Kenichi Reddit:
Heart seems to be a word that has multiple meanings:
- Courage (Chp 384)
- Belief in one's own logic a.k.a conviction (Chp 398)
- Compassion (chp 259-261)
- Mercilessness/emotionlessness (chp 259-261)
I'm not sure I got all the meanings. I think tenacity may be one of them. I use to think that ki was equal to heart, I thought it was another word for the strength of the heart, but I've since changed my mind.
Lugh was said to be the yomi with the most or the second most ability with ki, yet Kajima said that Koukin was the one with a stronger heart than him. Suggesting that only Koukin had more heart than him in Yomi. So Lugh doesn't have the strongest or second strongest heart in Yomi, or at the very least at the time Kajima mentioned Koukin, Lugh didn't have the most heart.
On the HSDK wiki it says ki is will power. What do you think is there a correlation between strength of the heart and strength of ki?