Most people donāt see this side of Jonathan because their first exposure to Phantom Blood / JoJo as a whole is the anime which ( for WHATEVER reason ) cuts out several scenes and details that enrich the story and the characters.
There were scenes of Jonathan being depressed but most of them never made it into the anime. If you look at the manga heās honestly so mentally unwell. You donāt see it because heās always presented as this strong, determined and unwaveringly noble figure but heās pretty messed up.
He contemplated whether or not that anyone would mourn over his corpse WHEN HE WAS 12 YEARS OLD. He was already feeling inferior to Dio and like he didnāt matter to anyone.
He outright SAID that he was depressed after he came to terms with the fact that he would never be able to repair his relationship with Erina after what Dio did.
All of THIS is a culmination of everything that Dioās done to try to break Jonathan over the course of his time with the Joestars. Jonathan beat Dioās ass for what he did to Erina, yes, and Jonathanās willpower is INCREDIBLE but Dio still very clearly left an impact on him that could only have gotten even worse with everything that happens after this ( finding out about Dannyās death, George Joestarās death, Dio turning into a vampire, the Joestar Mansion fire, having to fight Dioās zombies, Zeppeliās death etc. )
Even the way he mourned Danny seemed veryā¦.. real for someone with depression. Speaking as someone who struggles with severe depression and has lost a beloved pet of mine very suddenly, my grief manifested a lot like this.
Itās like Jonathan has been through so much misery because of Dio and has been so down that he couldnāt even process Dannyās death properly in the moment. He just mechanically went to his room and slept. Then at a random moment, when he was reminded of Dannyās death and the loss of his only friend, he wept uncontrollably in the privacy of his room.
He hasnāt expressed these emotions with ANYONE, not even his own father. I think he felt like he had to keep all of this grief to himself. Despite knowing that Dio was the cause of Dannyās death couldnāt get closure or justice even if he tried since everyone loved Dio and thought that he could do no wrong.
Something thatās glossed over is the fact that Jonathan spent seven whole years growing up lonely, which I can only assume is because of Dio secretly sabotaging his social life. Just because Dio decided to play friendly with Jonathan doesnāt necessarily mean that heād stop trying to spite him in more subtle ways.
This type of isolation can do things to people.
Sure he was seen as a beloved hero of Hugh Hudson Academy for his intelligence and athletic ability but I donāt think he ever had any real friends there, or at any point in his childhood. No one except for Speedwagon went to check up on him after he was injured in the Joestar Mansion Fire. He never mentions any notable friends from his days before Hamon either. None of them even saw him off when he was going on his honeymoon with Erina. The only ones that did were the Hamon Masters, Poco and Speedwagon, who heās only known for literal weeks at most. In his normal life we only ever hear about Jonathan and Dio hanging out often to the point where their classmates would call them ābest friends.ā And their relationship is another can of worms entirely.
The first real friend he got after Dannyās death and Erinaās assault was Speedwagon, who was a street thug WHO TRIED TO KILL HIM.
Also Jonathanās classmates apparently described him as being quiet. Iāve also seen translations use the word āmild-manneredā or āsoft-spoken,ā which is NOT how he was when he was younger / still able to enjoy a pre-Dio life. š
We can only speculate what Jonathanās life was like during the 7 year timeskip but I legitimately think that he was depressed even if he pretended not to be, just like how he pretended to be friendly with Dio even if deep down his heart couldnāt fully embrace him as a brother. And I feel like all the trauma he experienced in Phantom Blood after the fact only made his depression even worse.
Thereās also something suicidal about the way Jonathan goes about doing heroic deeds and sacrificing himself. Itās selfless, but itās the type of selflessness where he disregards himself and his own well-being in favor of saving or protecting others. We see it constantly throughout Phantom Blood, ESPECIALLY during his death.
He cares a lot more about others than he does himself. Itās definitely his nobility and golden spirit at work, but I also think a part of that stems from a lack of self-worth because of how critical George was of him throughout his childhood and how much he was compared to Dio.
During their honeymoon Jonathan also confessed to Erina that his heart was ācold and darkā and that she was the one who gave him his warmth back. Itās a testament to how important Erina was to his happiness, but I also think that itās SO telling about where he was mentally, especially when he spent the majority of his life SEPARATED from Erina because of Dio and his obligations as a Hamon User.
TL;DR: Jonathan Joestar is a more complex and broken character than most people realize. Also he DESPERATELY needs therapy.