For Odd:

He was very inquisitive and polite as a young boy (albeit a bit socially awkward), but around age 13 he was a bit more privy to some of the scarier/mind-bending stuff to his past and the circumstances around him, so around that time, so he tried to cover up his increasing mindset of "My god, what is happening?" with a veneer of humor (with varying degrees of success)

By around age 17-18, when his father died: He did NOT take it well. By this point, he had already taken on some of the responsibilities his connection to Giratina came with, but he grew a bit more spiteful and vindictive in his practices: never intentionally trying to hurt anyone ((most of the time)), but certainly more callused and careless with both his words and his actions.

And then a year or so after that, with a lot of things in his story coming to a head (in a rather unfortunate way), Odd was left pretty shell-shocked in the aftermath and decided to get away from it all for some DEEP reflection, hence why he's hole'd up in his shop. Nowadays, he's much more mellow and used to the strange stuff that happens to him, but he's still sifting through a lot of baggage

Elio was originally a quiet and emotionally-stunted child who's maturity peaked much earlier than his other younger siblings due to his involvement with a certain Dark-type Mythical Pokemon whom he viewed as his second parental figure. Also, Elio was heavily sacrificial, putting his siblings needs over his own because of the early realization how his older siblings had gradually changed after the death of their parents, with Ardyn unintentionally bombarding Elio with the importance of family and what it meant to be an Argentium now that they're being raised by their grandfather - one of the most powerful people in the region, let alone the world because of his past contributions. Thus, he was generally agreeable, obedient, and well-mannered.

After meeting Molesurya - his first Pokemon, and Adiunna - the former's mother and Elio's own maternal figure that, Elio started to develop his emotions. Not to mention the constant exposure to other's dreams and nightmares via said mythical's powers caused him to learn and constantly self-reflect. Inevitably, after encountering the "final dream" via his grandfather and learning of the loneliness he went through via alienation from his grandchildren in their misguided attempt to be self-reliant and not burden him, that Elio becomes more open with his emotions and desires while adopting much of his grandfather's carefree and amicable personality - which he would use as a foundation to fully develop his own personality over the coming years.

By the start of his journey, Elio's fully developed into the jovial, semi-altruistic, pragmatic, prodigiously, hard-working, incredibly wise and equally charismatic individual who's also noted for his occasional snarky, teasing, and provacative attitude with those he truly considers to be his friend or ally. Such a multi-faceted personality is usually seen in various OC interactions and fanarts I commission of him.