Despair and Romanticism within the story of Jinyoung.
When I was rereading HFW I noticed some great parallels alongside a deeper conflict which is being emphasized within the Gapryoung Kim arc. Since a lot of people might not realize the writing involved I decided to emphasize it.
Jinyoung's optimism/romanticism as shown within the Gapryoung Kim arc relied almost entirely on Gap. Gap proved time and time again that his overcoming/romanticism will always win against his opponents (specifically the Yamazakis who are meant to symbolize despair within the arc).
Jinyoung is someone who in the very beginning of the flashback is overwhelmed with despair. He is in despair before Gap shows up. When Gap overcomes and defeats these opponents of Despair, Jinyoung is retroactively affirmed in his innate optimism and romanticism.
But when Gap died to realists like Elite and another symbolism of Despair, Kitae. Jinyoung's entire worldview was shattered. He could no longer believe in romanticism as his ideal of romanticism was killed off without being able to overcome like he always did. Gap who was in way a figure of Jinyoung's hope was killed by the people (specifically Elite) who never embraced romanticism. Jinyoung goes into a mental breakdown with despair as he loses sight of his optimism and conviction.
We see Jinyoung in constant rage and self loathing both in 1A and in HFW. To him his failure to protect anyone he cared about could only be blamed upon an external source, the realist Elite. Jinyoung again externalizes, this time it's his despair which he blames on Elite specifically.
It is only when he realizes Daniel (his nephew) is about to be killed in HFW he takes hold of the conviction he had and justifies it via himself without the need of an external figure to hold to. He does not need the idea of Gap and the burden of defeating Gap's killer (Gitae) because he has successfully established romanticism for himself in essence bending the world to his own will of protecting his nephew to the end.
Combined with Kitae being stated to be the the personification of Despair. It makes sense that Kitae served as a symbolic representation of Jinyoung breaking the fear of despair by himself and fighting till the bitter end with it, no Gapryoung Kim required.
And to show Jinyoung's maturation even though Kitae defeats him, he does not break down mentally and is not consumed by despair.
You could also reconcile Jinrang's statement due to this. Jinyoung was in essence imitating Gap's overcome. Though yes Jinyoung had a copy that could copy unique skills and even overcome to a similar extent. Jinyoung did not instill overcome into his very essence.
Jinyoung could copy a overcome punch but he wasn't overcoming. Not until he achieved path and made something of his own for the first time was when he succeeded.
Since established Jinyoung placed romanticism within himself, Jinyoung started to overcome for the first time (as seen in the panel). His act of protecting Daniel led to him overcoming and though he lost against Gitae he wasn't overcome with despair as his very essence was now filled with romanticism.
I really do think PTJ has a lot of room to navigate this romanticism/Despair parallels. Though since he does it in such a implicit way it'll not be picked up by half the fandom.
TLDR – Jinyoung's optimism was reliant on Gapryoung. When Gap died to realists (Elite) and despair (Gitae), Jinyoung's extrinsic romanticism was destroyed. In HFW, by protecting Daniel he finally internalizes romanticism. Jinyoung despite being defeated by despair doesn't lose his romanticism either.