Serena is obviously kind and friendly, she is very hopeful at the same time as she lets her imagination take over her, she is realistic and accepts that people have their own time, for example after she met Ash again (and almost called him in front of the GIRL gym battle LOL) thanks Clemont and Bonnie stopped her and asked her what she was doing LFMAO, she waited after introducing herself and gave Ash time to approach him at the right time to try to say what she came to say (People totally misunderstand her so much, she never "threw herself at Ash".
At most, you tried to get his attention, which didn't even work out so well, LFMAO, and that's what's funny about everything, because despite everything going wrong, things also went right at some point.
I love her relationship with her Pokemon, how she grew as Fennekin progressed, how Fennekin was the one who introduced her to a battle, over time she started sending them to help on her own.
About how she was the only one to understand Pancham, and simply threw herself at a Shadow ball to protect Fokko/Fennekin I would have liked to have seen more than Braixen and Pancham fighting over his character tbh
Eevee she was so...my god, it was one of the most beautiful stories in the Pokémon word, all the patience that Serena showed was impactful, as a semi or non-verbal autistic Eevee is so me, especially the fear of popping noise reminds me of my own sensory sensitivity of the party ball being popped, 😭💕💓
I felt so seen and represented when everyone gave everything to find her and Serena was able to just miss her own performance for her Pokemon.
😭🩷✨
I love them ALL

Well. There are so many things about Ash and Pikachu that come to mind when talking about their characters, Pikachu is in every way Ash's best friend, he is the one who spends the most time with Ash and knows him more than no one else and the opposite is also important to be shown, I feel like people see what Pikachu does for Ash but don't see what Ash also does for Pikachu (aside from the saving Pikachu life thing), about how they are individually, about how Ash listens to everything Pikachu has to say, about how Ash cares about how Pikachu feels, about how he cares about the little things in his day I believe that many series don't convey this much outside of the original series, Shudo was great at that...me AG Ash and Pikachu are perfect, Ash's Pokemon interact with Pikachu and each other, the team's Pokemon interact with May's Pikachu and Ash, sometimes the characters ask Pikachu or talk to Pikachu when he isn't there Ash's shoulder or when he is..., DPAsh and Pikachu are an inseparable and unstoppable duo ✨😭 I love how everyone talks about Pikachu and Ash but at the same time Pikachu being his own self, people interested in Pikachu as a character (like Paul, despite not respecting Ash, he had great respect for Pikachu's power to the point of staying in Ash's gym battle because of Pikachu and Pikachu and Ash bound's determination)
I hope this helps and gives you some inspiration.
About the Headcanon part...I don't know Maybe I'd like to hear more about Headcanon from others, I don't know much about the fandom in general, I like creating my own stuff.
But I would love to see more Headcanon about Ash and Pikachu

Okay, this is a difficult question to answer...
I never thought much about it, I think liking his character is so natural for me, I think everything, absolutely everything, how he gives himself to people and Pokemon, how he helps people through the way, how he changes the lives of Pokémon and trainers in the places he goes, not just his Pokémon, that's why I like episodes of the day (or fillers with him) so much, I like everything he's in engaged in the moment
Of course I have my problems with the series like everyone else
But I love his empathy with people, that's what he taught me the most, If I am who I am it is because of his kindness and perseverance, whether impatience or impact the way he is...it is perfect 💕✨
I like his sarcasm, I like it when he shows angry emotions or is just upset because of some problem, that's what makes him human and relatable
I also like it when he's silly and sensitive, he needs to keep up with his nonsense to look like Ash we know despite others complaining, I like it
Taking away which Ash is creative for combinations and movest is the biggest crime that his haters want 😭 I still don't understand this mentality, like how people want him to never do the strategies of battle style HE IS SMART AND DESERVES ALL THE LOVE IN THE WORLD