ok i'll give this a go .. and i'll make it hard for myself i'll chose the tsukihimi comic strip one where it's in black and white so the colour difrences have been removed and it's drawn in a simplified style so it leaves out a lot of fine detail
here we see hisui akiha and ren
Hisui : the tops of her eyes are flatter than most combined with uncurved slighly dead eyes note her personality here eyebrows are useually arched high like this when she is flustered adding a certain amount of cuteness (she is a fan favorite)
her hair style is of course very distinctive
akiha : her eyes are more curved than hisue but are drawn very shap and curving down towards the center showing her iratable personality her eyebrows are genraly close to the top of here eyes of drawn with a kink in them showing her creasing her forehead in frustration again her outfit and hair is quite distinctive ... but she does have incho hair ... the hair band is a dead give away also her face is genrally given slightly sharper angles for the same reason as her eyes
Ren : ren is always drawn quite sofly with many lines light of blured as she is a cat deamon many of her fetures portray that including her pointed ears and the indentaion in her eyes. her eyes have also got much larger boders around them this is the simplified way of drawing longer eye lashes etc her eyebrows are again placed high and sligtly arced her mouth is darly shown as she doesn't talk and again her hairstyle is distinctive . her face is also often quite rounded to show here child like apearence .. this is combined with a semi-perminant slight blush on her cheeks (this is manga trait for child like charactors)
also yes the dudbbed voices are crap and in no way should be listened to .... the japanese voice actors are much beter and really give life to the anime
the yellow remark was just to signify that cartoons don't nesaceraly look much like what they are representing and just because you can't tell the difrence doesn't mean there isn't one
also with the "big-eyes" thing it should be noted one of the reasons it was copyed was because a lot of information about personality and emotional state can be transphered via the eyes