The 3 starters are getting focus, Oshawott, just stands out more due to his personality.
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Basically what I'm saying -
Oshawott - Comic/Personality/selfishness/Most fillers/If you do use the game comparison - Bianca - Fire, Trip - Grass, Ash - Water
Tepig - A lot of Main Battles/Gym Battles/Favorite for use in battles/If you do use game comparison - Pansage and Tepig are both in the Gang's group. Only Starter singled out for his type.
Both - Used together a whole lot, Gang talks about them a lot, both have awesome personalities, but Tepig is 10x cuter due to not being a show off, does have a little bit of comical in him as well. Weakest rivals of Ash&Friends have the Fire and Water starter. Both of their evolution lines have had terrible treatment.
Battle/Move development is huge throughout the Anime for both.
Snivy on the other hand has had the feel of getting left behind the 2 of them constantly, but on the one hand has been like an "Older Sibling" to the others, making this so confusing to begin with... -.-
Did I miss any?