INTRO
What I'm about to say may apply to pretty much any model, including of course Pony.
The reason for talking about Illustrious in special is that it has access to lot more characters by default than other models.
Illustrious Is a model that already knows lots of characters, so you don't really need a lora to pull many famous, and no so famous, characters with illustrious.
However, the use of lora can help to get most accurate depictions of some of the costumes, stabilize a bit the style and also helps to easily remove and add particular pieces and elements from the outfits.
When generating, there are several options.
Not using a lora and generate the characters, for this you will need to MENTION the character*.
Using lora, trigger the lora (if trigger word required for that lora) and NOT mentioning the character.
Using lora, trigger the lora (if trigger word required for that lora) and MENTIONING the character.
Any option will work and in the end it is up to you which one you prefer, but it is important to know, that naming the character on top of using a lora will change the results considerably.
NOTE*: Please, take into account that some characters have so much entries that only by including their traits may led to the model to generate "them" even if you do not name said character. As you can see below some of the prompts dont use lora or mentions the character and still the likeness is very hight. But thats usually not a reliable method.
And happens on the oposite too, some characters just mentioning them won't be enough, and you will need to include some traits of the characters as well to make the model pull them.
EXAMPLE 1
BASE PROMPT:
1girl, blonde hair, long hair, blue eyes, forehead, sidelocks, braid, keyhole, forehead mark, polka dot bow, hair bow, parted bang, grey dress, long sleeves, skirt, bodystocking, gloves, beret, bare legs, holding lantern, serious, night, hallway, mansion, night sky, window, indoors, cowboy shot, blushing
PROMPT A
<lora:Abby_Illus:1>
PROMPT B
abigail williams \(fate\), abigail williams \(event portrait\) \(fate\) <lora:Abby_Illus:1>
PROMPT C
abigail williams \(fate\) <lora:Abby_Illus:1>
PROMPT D
Nothing was used in this one
PROMPT E
abigail williams \(fate\)
PROMPT F
abigail williams \(fate\), abigail williams \(event portrait\) \(fate\)
CONCLUSION
As you can see, to get that particular costume more accurately, it's better to use the lora only without mentioning the character.
ANOTHER EXAMPLE
PROMPT A (Lora, character not mentioned)
2girls, indoors, ruins, post-apocalypse, 1girl, dark skin, pink hair, orange eyes, barbarian, fur trim, (mohawk, shaved head), sitting, couch, spiked collar, spiked bracelet, shorts, belt, sweat, midriff, groin, dirty, fingerless gloves, looking at viewer, licking lips <lora:Kuro_Illus:0.8>
BREAK
2girls, indoors, ruins, post-apocalypse, 1girl, very long hair, parted bangs, blonde hair, blue eyes, small breasts, goggles on head, scarf, cloak, torn clothes, shorts, belt, tank top, holster, thight strap, partially unbuttoned, thighhighs, armored boots, gloves, knee-pads, elbow pads, dirty, thumbs up, smile, close-up, forehead <lora:Abby_Illus:1>, masterpiece, best quality, amazing quality, very aesthetic, absurdres, highres, newest
PROMPT B (Character mentioned, not lora)
2girls, indoors, ruins, post-apocalypse, 1girl, dark skin, pink hair, orange eyes, barbarian, fur trim, (mohawk, shaved head), sitting, couch, spiked collar, spiked bracelet, shorts, belt, sweat, midriff, groin, dirty, fingerless gloves, looking at viewer, licking lips, chloe von einzbern
BREAK
2girls, indoors, ruins, post-apocalypse, 1girl, very long hair, parted bangs, blonde hair, blue eyes, small breasts, goggles on head, scarf, cloak, torn clothes, shorts, belt, tank top, holster, thight strap, partially unbuttoned, thighhighs, armored boots, gloves, knee-pads, elbow pads, dirty, thumbs up, smile, close-up, forehead, abigail williams \(fate\),, masterpiece, best quality, amazing quality, very aesthetic, absurdres, highres, newest
PROMPT C (Lora+character mentioned)
2girls, indoors, ruins, post-apocalypse, 1girl, dark skin, pink hair, orange eyes, barbarian, fur trim, (mohawk, shaved head), sitting, couch, spiked collar, spiked bracelet, shorts, belt, sweat, midriff, groin, dirty, fingerless gloves, looking at viewer, licking lips, chloe von einzbern, <lora:Kuro_Illus:1>
BREAK
2girls, indoors, ruins, post-apocalypse, 1girl, very long hair, parted bangs, blonde hair, blue eyes, small breasts, goggles on head, scarf, cloak, torn clothes, shorts, belt, tank top, holster, thight strap, partially unbuttoned, thighhighs, armored boots, gloves, knee-pads, elbow pads, dirty, thumbs up, smile, close-up, forehead, abigail williams \(fate\), <lora:Abby_Illus:1>, masterpiece, best quality, amazing quality, very aesthetic, absurdres, highres, newest
Negative prompt: lowres, worst quality, bad quality, bad anatomy, extra fingers, sketch, jpeg artifacts, signature, watermark, old, oldest, 3d
PROMPT D (nothing)
2girls, indoors, ruins, post-apocalypse, 1girl, dark skin, pink hair, orange eyes, barbarian, fur trim, (mohawk, shaved head), sitting, couch, spiked collar, spiked bracelet, shorts, belt, sweat, midriff, groin, dirty, fingerless gloves, looking at viewer, licking lips
BREAK
2girls, indoors, ruins, post-apocalypse, 1girl, very long hair, parted bangs, blonde hair, blue eyes, small breasts, goggles on head, scarf, cloak, torn clothes, shorts, belt, tank top, holster, thight strap, partially unbuttoned, thighhighs, armored boots, gloves, knee-pads, elbow pads, dirty, thumbs up, smile, close-up, forehead, masterpiece, best quality, amazing quality, very aesthetic, absurdres, highres, newest
CONCLUSION
NAMING the character on top of using the lora can lead to the character retain some elements you may not desire, because in the model database (which activates when you MENTION the character) is different and sometimes more strong than the lora and will "overwritte" it.
So, mentioning the character may produce better results depending the case, but it will be, more often than not, harder to control than using lora.
For example, prompting "abigail williams \(fate\)" will often add bows everywhere, and you may need to add them to the negative prompt and even then sometimes they will still be there, meanwhile, if you dont mention them and use a lora they usually simply won't appear.
tl,dr
You DO NOT NEED to use lora for generating certain characters with Illustrious model (this also apply to Pony or Animagine, but Illustrious knows more characters by default). BUT depending on what you want to do it, you may still need a lora.
For example, if you want the character in a swimsuit, you do NOT need a lora, just mentioning the character and asking for her in a swimsuit will be enough.
BUT if you want that character in a very specific alternate outfit, you still may need a lora to get it.
Also, with character lora is easier to "remove" particular pieces of clothing and accessories than if you mention the character because mentioning it certain elements are way too embedded into the database.