The war that Achillies faced was the longest campaign that any chosen hero had to go through lasting almost 6 years (give or take Loki's time bullshit but he wasn't awake for most of Ganon's reign and Termina took a long time but not 6 years worth). That's to say, a lot of things happened during that time.
Loki and Tingle appeared a year in, Chulainn was brought in 2 years in along with Medli, King Daphnes and Tetra, Marin came in halfway through that year, and Ravio and Yuga was summoned on the third year.
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Ravio would grow quite the spine out on the field. He originally started as a support, running to and fro to deliver supplies and aid the wounded, but after some time he started to fight back, generally when the kids he'd later place under his charge were fighting on the front lines. Marin did much the same, though she had a spine to begin with, and the two would become fast friends. The troops would often refer to them as the Husband and Wife with how they often teased and bickered. Taking care of the younger ones didn't help.
Marin, though she had a backbone, had started off weaker than others. Sure, she had what was basically a god on her side that would aid her when summoned, but beyond that she was of little help besides giving support. She had no experience after all. Even Ravio had some history with weapons, crafting them, testing them, and apparently even having received a bit of training when he was younger. Medli had some too as all Rito received some training (not to mention she was something like a Sage, whatever that was). Still, her buffs and debuffs were useful and so she continued to aid thoes around her the best she could. Ravio was a good complement to her, as with her buff, his hammer and magic became quite a force to be reckoned with. Though sometimes she felt like he was stronger than he let on, but maybe it was limited like hers.
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As time passed people aged, Chulainn, Medli and Tetra became visibly older as they were entering their teens or reaching further into it. Everyone changed, as people do through time. Everyone but Loki. See, Loki could smell some time-bull shit going on and knew once they returned back to their eras, they likely would revert to their original state of body too. He knew personally how jarring and hard to cope going back several years could be, so he used his Fierce Deity mask every three days to turn back the clock on himself, making sure he never aged more than 3 days.
Of course this is just what he told everyone. In reality he was scared of growing up and changing. He'd lose everything he loved over and over and he wouldn't be who he was. He'd seen it before and it was terrifying. Some hero of Courage, huh? But when Ravio had basically adopted him, he found that you could grow up, change, and still be a kid at heart. Ravio and Marin were perfect examples of this after all. They would snicker at fart jokes, make weird faces and mimic each other poorly, do what could only be some slap-stick comedy routine that involved lazily slapping each other, and still do all the things that made an adult an adult... or at least they did from what he could tell. By the end of it he wasn't using the Mask as often, though his original statement still stood so he did use it every week instead.
He would very much grow up to be a strong adult with a heart of a child. continuing his pranks and silly faces. Even when training the next hero, he'd sometimes throw in a joke that the Hero of Twilight would visibly cringe at. Just because your body changed and your experiences changed you doesn't mean you have to leave behind all that made you, you.
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Chulainn had largely established the fact that he was fine with everything from the get-go, but it would build up and break down his walls at some point. Unlike Medli, who often sought comfort from others despite her 'older sister' persona, and Tetra who was captain of a fleet of pirates (who probably had her own coping mechanism), the only people who had helped him get through his initial adventure was his Grannie and the King of Red Lions. But grannie wasn't here and he was afraid of growing close to King Daphnes. He had to say goodbye once and it hurt; he didn't want to do it again. If he got close, he would have to.
The damn would break mid-battle of course, because why the hell wouldn't it? It would happen when he was surrounded by strangers he was only starting to get to know, but all of them were adults who thought a kid couldn't do anything, even if he could beat them all in battle and a game of poker. Then Marin appeared and defended him while Medli calmed him down enough for them to retreat to base for a bit. He was really embarrassed by it, but Marin assured him its natural for anyone, especially one so young. Any grown adult would be quaking in their boots in his situation and would have broken long ago.
When the battle was over Marin told him a truth that none of them liked. She would likely disappear for good once everything returned to how it was, for she was just a dream. Unlike real memories, even lasting dreams fade into nothing after a while. The memory of her would become watery with time until she faded completely. He realized that despite the heart break, he'd have to say goodbye to everyone here at some point, and it would hurt just as much. He should enjoy the time he had with them, even with the King. But then Ravio popped up a few months later and said that if a dream had a strong enough of a presence that it would last in their memory for years and years to come, just as the Koholint Island did with his hero.
Yes, many things happened during the War of Eras, a long 5 years (as Achillies would insist but everyone agrees it was closer to 6). Some things bad, some things good.