It just takes time to get them introduced.
We're in the same situation as you, our kitten (Willow) looks just like yours too, though she's 4 months. We have a 1 and a half year old cat Baggy(Bagheera) who's the current king of the house.
Keep the kitten in a 'safe' room to start with everything they need. We keep Willow in our bathroom with everything she needs, we let her wonder around the house while Baggy is shut in another room, then shut her away and let him wonder around, so they could get used to each others scents. Start slowly introducing them, they'll hiss and growl at each other and may take the odd swat but it's pretty normal. We had a week of this, Baggy kept chasing her and cornering her and you sort of have to let them work out who's boss but make sure it's supervised in case it goes too far. Feeding them quite close together helps them associate each other with food which helps, try getting them to both play with the same toy too.
Ours are pretty much there now, he chases her, she chases him but occasionally he'll go too far and pin her and she'll start crying with ears back, that's when we intervene and let them cool off for a bit.
It's odd because she acts like she's terrified but after letting her out again she's running down to see what he's doing again. He's a lot bigger than her though, so he could be taking it too far and hurting her. You just need to pay attention to body language to tell when they're playing and when they're fighting.
Pics below of ours
Side note - Make sure you get him de-flea/vaccinated ASAP, especially if he's been outside so young. The previous owner of Willow was awful, rubbish cheap food, rubbish cheap litter and she had fleas when we got her, they didn't look after her. We took her to the vet, got her on Advocate and she's all good now.