Well, from the looks of things you're setting a good part of game in the subway system/underground areas as far as I can tell, so that already gives a fairly decent setting when it comes to placing such gribblies.
You can perch them upside down, on overhead pipes in shadow, waiting for the player to pass on by so they can swoop down. To add some flavour to this, you could have two kinds of bats (maybe with a very subtle difference in their sprite... maybe add male and female coloration
) that both have different attack patterns (eg one has a low swoop, while the other does a 'fake-out' dive before coming in low). It's up to the player to learn to tell the difference in coloration, and to then react accordingly.
...Also, two different enemies using the similar sprite means less workload if you wish to pad out the content a bit more.
Alternatively you could go for a flying enemy that swoops in from the side of the screen after a warning screech (echolocation), and (I'm not a programmer so I don't know if this is even possible) if you could make it so that the bat/bats (this is a chance to use the same/similar sprite to create two versions of the same enemy with varied animations depending) only attack the player if
the player is moving then it would be a pretty unique enemy type.
The player would be forced to choose between standing still and avoiding the bats' attention completely (you could probably explain their lack of sight by a 'mutation did it' excuse if you're doing a lore/game character pedia in game) or to dodge any incoming ground-based enemies, if needed.
Now, as for actual sprite design, I'm not an artist (I know, I'm not very helpful when it comes to game design, out of all the creative paths I could have picked I went with writing), but going off of the sprites you already have I don't think you'll need much help with that.
Animations wise, you have a variety of options, and I'll list some of the ideas off of the top of my degenerate mind:
1. Long tongue + latching onto chest = The frenching of a life-time (Good for a warm-up/non-ko attack if your going for them.)
2. Moterboating/shirt tear and boob suck (throw the boob-lovers a bone). Once again, could also be used as a 'warm-up'
3. Latch onto back + blood suck from neck + titty touching with one wing hand + groping down below with the other can then lead to the protag collapsing down to her knees with combined pleasure/blood loss, setting her up for a sex assault/full on fruitbat face-fuck. This will probably be a bit too long, but you could always implement it as a 'Game over' scene.
Actually, on that topic, how are you planning to structure the animations? Will each enemy have one or two 'attack' animations and then it'll loop the last one upon player defeat, or are you going to chuck in a specific 'game over' animation loop that plays when an enemy reduces the player to 0HP?
Also, sorry for the long-winded post (actually I shan't apologise, decent games with bat enemies that actually H-attack are pretty much non-existent), but here are also two games with bat enemies that you could possibly use for a bit of inspiration.
Hounds of the Blade
Twilight Detective 2 - warning, this one is pretty shit
That's what you get for asking me for suggestions.