There is a common misconception regarding Leo De Luca: that you must get him in play early for him to be effective (preferably in your opening hand) and that a Leo De Luca drawn mid or late into the game is useless.
It is true that the earlier you get him, the more extra actions he will give. But the opportunity cost for resources is much higher earlier in the game; spending 6 resources will prevent you from getting other assets out into play, and without other assets or events the effectiveness of each of action isn't very high. It's hard for a rogue to investigate without a Lockpick or Flashlight or to fight without a weapon, so much of the time you'll be using his free action in inefficient ways such as drawing cards or gaining resources.
In contrast, if you put him into play later in the game, the actions he gives can be used much more efficiently. Each Investigate can with a Lockpick is nearly guaranteed to succeed, and can trigger a draw from Lucky Cigarette Case. An evade gives you extra stuff from Pickpocketing. Use the action he gives to play Hot Streaks and Intel Reports. Or it could be as simple as your skill tests being more likely to succeed due to having static bonuses out. The opportunity cost for 6 resources is also much lower in the mid to late game, since tend to over-invest in resources anyway to increase deck consistency (and to run Well Connected/High Roller), and there are few better ways to spend those resources than to get 3-4 extra actions and a 2/2 soak.
So mulligan him away for your Lockpicks, your Ornate Bows, your Berettas, and play him when money's no object. Starting with only Leo tends to be an expensive trap (unless you lucked into Leo along with Hot Streak AND your key assets, but what are the chances of that?).
The real cost is still the opportunity cost of an ally slot. At 0xp the extra action will be difficult to efficiently use, consider other allies to increase action efficiency or give static boosts. Once you've gotten the core of your deck and all the money you can't spend, buying a Leo (1) with Charisma is always worth considering if you've got the deck slots to spare.