Back when I was still collecting and playing Yu-Gi-Oh and bought loads of cards from GameTimeCC and TrollAndToad, I noticed TnT also had Pokémon cards. At first, I only bought the cheapest Venusaur, Nidoking, and Hitmonlee they had (the only three gen 1 Pokémon I was missing in my childhood’s collection, to complete the first 151).
About a year later, I placed all Pikachu they had in stock below 100 USD in my cart and decided to start collecting all Pikachu TCG cards. Collecting has always been my passion (I’ve collected dozens of different things in my life, starting with pretty pebbles and post stamps as a 5 y.o.). The reason why I decided to collect Pikachu (even though Pikachu isn’t even in my top-25 favorite Pokémon)? Simple: there were a lot and it would keep me busy for a while. 
Well, that certainly was an understatement.
Within a few months I decided to collect all languages, and came across Justinator’s collection here on the forum, whom at the time had ~550 out of ~650 existing Pikachu TCG cards. His collection contained Pikachu cards I had never even seen before, and it was a real inspiration. This was all in 2015/2016. As a short summary: in 2018 I surpassed him and had the largest Pikachu collection in the world (a.f.a.i.k.). Early 2019 I was surpassed myself by a collector from Belgium (Tom). In 2020 I was struggling to keep up with all the Pikachu releases, and also increasing prices due to the pandemic. In 2019, a total of 233 Pikachu TCG cards were released across the 11 active languages… And near the end of 2020, even simple English McDonald’s promos went for ~50 USD instead of the 5-10 USD they were in previous years. At that point I decided to stop collecting all languages from February 27th, 2021 (25th Pokémon Day) onward, and only focus on English and one of each non-English exclusive artwork. (And of course still the missing cards from before that date in all languages.)
Mid-2024 I was able to complete the English portion of my Pikachu TCG collection. And right now I’m only missing nine more Pikachu cards (from the total 1433) before that date: two German; two Spanish; and five Japanese.
Most of my Pikachu TCG collection can be found in the links at the bottom of the first post in my collection thread.
As for Seviper: it’s my favorite Pokémon. I’ve always been fascinated by snakes 
as a kid, and although some of the other snake Pokémon are cool as well, like Ekans, Arbok, Onix, Steelix, Serperior, etc. Seviper’s design is on another level, so the moment it was released it was my favorite Pokémon.
I started that TCG collection in all languages including cameos (unlike Pikachu) back in 2017 or 2018 or so, and completed it in 2022 iirc after finding the Japanese unlimited edition WCP Seviper and Battle Frontier cards. After that it was uncomplete again for about a year, but I recently completed it again after finding all missing Asian (Korean, T.Chinese, Thai, Indonesian, S.Chinese) Seviper cards.
Nowadays I primarily focus on non-TCG Seviper stuff. Basically everything that features a Seviper (unless it features the entire gen 3). Pushies; figures; stickers; non-TCG cards; posters; DVD covers; manga; pogs; tags; magnets; etc. etc. I even have a cushion and matching bedsheets.
Most of my non-TCG Seviper collection can be found in this post of my collection thread.
Greetz,
Quuador