You can befriend the following students at Hogwarts; when you have achieved a Level 10 Friendship level with a particular student you will unlock a customization item particular to that student.

Andre Egwu: Quidditch jersey
Alanza Alves: Alanza's Robes
Badeea Ali: Badeea's pants
Barnaby Lee: gray house sweater
Beatrice Haywood: Beatrice's Hairstyle and Hat
Ben Cooper: Ice Robes
Bill Weasley: Weasley sweater
Cedric Diggory: Cedric's outfit
Charlie Weasley: Charlie's Hair
Chiara Lobosca: Chiara's Necklace
Corey Hayden: Corey's School Outfit
Diego Caplan: Diego's denim jacket
Dobby: Mismatched Outfit of Clothes
Erika Rath: Erika's outfit
Fred Weasley:  Relashio Spell
George Weasley: Levicorpus Spell
Hagrid: ??? Mystery reward
Ismelda Murk: Ismelda's pants.
Jae Kim: Jae's hoodie
Levi Kidd: Levi's Hairstyle
Liz Tuttle: Liz's pants with sweater wrapped around the waist
Murphy McNully: White Shirt and Quidditch Pin
Nymphadora Tonks: Tonks hair (color not included)
Orion Amari: Orion's hairstyle
Penny Haywood: Potions Belt
Rowan Khanna: Rowan's glasses
Skye Parkin: Skye's Jacket
Talbott: Bird T-shirt
Tulip: Tulip's necklace
Victor Ketsueki: Jacket + shirt from Victor