I'm assuming you mean they have alliterative initials, not that all the characters' initials are the same.
Mostly they don't. e.g. Harry Potter, Hermione Granger, Ron Weasley, Albus Dumbledore, Sirius Black
But admittedly quite a lot of them do... e.g Luna Lovegood, Minerva McGonagall, Severus Snape,
There's no real reason behind this, and many people in real life have alliterative names as well.
Garrick Ollivander
A person initials are taken constructed by taking the first letter of their given first name and the first letter of their given surname and putting them together. For example, the initials of Harry Potter would be HP. Initials can sometimes include a middle name e.g. Harry James Potter becomes HJP.
Viktor Krum?
Ignotus Peverell
Ritchie Coote was a beater on the Gryffindor Quidditch team in Harry Potter and the Half-Blood Prince.
harry potter!
Helga Hufflepuff
Garrick Ollivander
There is no character in Harry Potter with the initials TS. There is a Sybil Trelawney initials ST. She was a professor who prompted Lord Voldemort to go after Potter.
A person initials are taken constructed by taking the first letter of their given first name and the first letter of their given surname and putting them together. For example, the initials of Harry Potter would be HP. Initials can sometimes include a middle name e.g. Harry James Potter becomes HJP.
Viktor Krum?
Harry's mother, Lily Evans.
Ignotus Peverell
D for Daniel and R for Radcliffe
Ritchie Coote was a beater on the Gryffindor Quidditch team in Harry Potter and the Half-Blood Prince.
Regulus Black from the Harry Potter series Rachel Berry from Glee Rubeus Hagrid from Harry Potter
That may refer to Angelina Johnson.