Most Helpful Guy
For me it's her choice of clothing and how she keeps herself and her appearance. I hate girls who rely on makeup and sh*tty clothing to attempt to look good, the vanity and need to look good for the guys will always broadcast that you care more about people looking at you than actually talking and having a relationship with you.
I like girls who keep going, look after their appearance but don't rely on clothes which show off cleavage, ass or curves, and who don't plaster their face with makeup to look good. You really see which girls could be false and vain and which girls are happy, healthy and can move forward in life.
The clothing too, has a big part of how to attract males, because opinions vary and fashion is a difficult thing to predict, as many people have different tastes. Hair in particular has a big impact on how good you look to a male or not, if it's a hairstyle he dislikes (Emma Watsons short hair style, for example) then you won't get anywhere.
There is really too many points to point out here. Try luring him into answering some innocent questions and then make subtle changes. Point out other girls and ask him what he thinks of them, if you have to. If he likes a girl, don't take it badly, just ask why and then take notes.
Have fun getting to know that guy you like! :)