Recommended Level: 6-7
There are no new species here.
When you arrive on Route 103, go north and east into the patch of grass and continue further north along the path from there until your meet up with Brendan/May. Feel free to battle the slightly-higher-leveled Pokémon here on the way to grind a bit faster.
Anyhow, once you reach Brendan/May, speak with him/her and he or she’ll eventually request a battle from you.
BOSS - Pokemon Trainer Brendan/May
Your rival will have a level 5 version of the Pokemon they chose at the start.
This battle, in all reality, shouldn’t be too difficult. For the most part, this lies in several reasons: firstly, unlike you for some messed-up reason, your foe’s Pokémon does not have a move of its type although yours does.
Secondly, it’s also because that same move they have doesn’t even have an advantage over you. And, third and most importantly, it’s because you’re likely a few levels higher than they are. You should be about Lv. 6-7 right now.
There’s no real particular strategy you’ll need to use for this battle; simply abuse your Normal-type move for now.
After the battle, your rival will heal your starter to full health and then head off for Littleroot. Follow him/her!