Lola Santiago is incredibly powerful in a Preston Fairmont deck. I’d say she’s an auto-include.
With Preston’s Family Inheritance Lola lets Preston discover one clue per turn on a shroud 4 or lower (5 or lower if he’s a Lonewolf...) without taxing his resource pool.
In solo, where many locations are single clue locations, this lets Preston solidly hoover up clues while tooling up / drawing cards / moving around the map with his three actions.
Lola is better than Leo in a Preston solo deck. She also gives you a free action a turn, albeit limited to investigating, but that action is test-less, and one you’ll often want to do every turn anyway.
Finally, how does she compare to Streetwise? Sure, streetwise is far more flexible, as it lets Preston boost either Intellect or Agility to 7 once a turn by spending all of his Family Inheritance. Lola boosts those stats too, albeit only to 2, but at least it’s a permanent boost. I’d rather be investigating testlessly than pumping my Intellect via Streetwise and still risking drawing an Autofail.
Streetwise’s main strength is that it’s permanent, so you really have to buy it first. But once you have I’d save up your next 6XP for two copies of Lola
Especially when you add in her damage / horror soak which effectively puts Preston up to 8 in both.