Which alt characters are permitted in names?

About none of them. I would have to look it up for sure, but last time I looked it was letters and numbers only.

: Honor please.

The character set depends on the region/language.

https://support.riotgames.com/hc/en-us/articles/201752814-Summoner-Name-FAQ

CAN I HAVE A SPECIAL CHARACTER IN MY SUMMONER NAME? Special characters are allowed in some Summoner Names depending on the selected language. If a special character is not utilized in the selected language, it will not be available to be used even if it is on a region that allows it.

Below are the special characters allowed for each region:

NA/OCE –

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

EUW/EUNE –

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyzàâÇçèÉéÊêëîïÔôœùûĄąĘęÓóĆ棳ŃńŚśŹźŻżÄäÉéÖöÜüßÁáÉéÍíÑñÓóÚúÜüΑαΒβΓγΔδΕεΖζΗηΘθΙιΚκΛλΜμΝνΞξΟοΠπΡρΣσςΤτΥυΦφΧχΨψΩωΆΈΉΊΌΎΏάέήόίύώΪΫϊϋΰΐĂăÂâÎîȘșŞşȚțŢţÀàÈèÉéÌìÍíÒòÓóÙùÚúÁ᥹ÄäÉéĘęĚěÍíÓóÔôÚúŮůÝýČčďťĹĺŇňŔŕŘřŠšŽž

Brazil –

0123456789 ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzÀÁÂÃÇÉÊÍÓÔÕÚàáâãçéêíóôõú

Russia –

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzАБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯабвгдеёжзийклмнопрстуфхцчшщъыьэюя

Turkey –

. 0123456789ABCÇDEFGHIİJKLMNOÖPQRSŞTUÜVWXYZ_abcçdefghıijklmnoöpqrsştuüvwxyz

LATAM –

0123456789ABCDEFGHIJKLMNÑOPQRSTUVWXYZÁÉÍÓÚÜ abcdefghijklmnñopqrstuvwxyzáéíóúü

Thanks guys!