Bair and utilt on their own kinda body most of this game's cast.
Pikachu likes to nair past opponents because while its nair doesn't have the best hitbox (making it more vulnerable to just being beaten out), Pikachu can more easily just cover a long horizontal distance in a quick shorthop. To counter this, depending on how low the nair is, you could lightshield grab or other quick OoS option, turn around to face Pikachu vs. the same OoS options, or just CC shine because it's great vs. most approaches. You can also roll through if you see Pika jump forward to gain stage position. Try to avoid aerials OoS, most Pikas will likely either get the uair out before you can hit with one of these or can otherwise maneuver around it.
On a similar token, Pika's OoS options have bad reach in front, but the quick uair OoS is has good range behind. Try to space out of the range of his sheld grab, watch out for the dsmash as you don't want it to launch you (some Pikas may try to do it out of spotdodge or CC).
Closer to the ground, a lot of Pikachus love their DD and dtilt. Dtilt ultimately isn't as good as Marth's and can be shield grabbed on shield DI in, and you should mostly expect Pikachu to try to DD grab or usmash approaches, so either overshoot, or do retreating aerials as Pika's range mostly pretty bad. Pikachu's ftilt can stuff a lot of approaches as well, so be wary of this move and try to either move around it or block and punish.
When Pika is offstage, it usually wants to go to the ledge, or if it can, may wish to go to a platform. Pika usually has a good bit of lag when landing on stage. Grabbing ledge and doing regular getup is usually desirable. You can try to intercept the path to the ledge and slightly above by dropping lower from the ledge with an aerial and double jumping back over the edge (ideally such that you can autocancel).
Onstage, you could think of Pika as a gimped mix of Fox and Marth to a degree, except that he's got no range. It's just that you can expect a lot of similar neutral options. Of of throws you can think of Pikachu kinda similarly to Peach as far as combos go.