To increase your affection with Alexis you would need to defeat Valravn.
In order to do this you would need to go near the beginning of the game and choose the Power option (recommended) when you first meet Michael over the Skill option, then when asked later in the game to choose the Horn, Mask, or Eye choose the Horn, then progress the game trying to gain as much Power as possible until your character meets H.E.R.O when they are about to fight Valravn (PAUSE AND CHECK YOUR POWER STAT, IT SHOULD BE AT 80, IF IT IS NOT AT 80 THEN YOU NEED TO GO BACK AND SEE WHY YOU DIDN’T MEET THE POWER CHECK). When fighting Valravn pick these options to beat him with no help from Brianna: Keep your distance, Go all out, I don’t need to evolve. Then you will beat birb.
I hope this helped and also here is the offical walkthrough: https://cdn.discordapp.com/attachments/833563117484113970/1539141264681992242/Official_Superhuman_Walkthrough.pdf?ex=6ac28a41&is=6ac138c1&hm=447b7f8ce0f69d287508367935214491eb5fdc514a28bd1a14b632780a7a9005&