[Kwarde]

pawn Код:

stock firstNameOnly(playerid)
{
    new Name[MAX_PLAYER_NAME], pName[MAX_PLAYER_NAME];
    GetPlayerName(playerid, pName, MAX_PLAYER_NAME);
    for (new i = 0; i < (strlen(pName)); i++)
    {
        if (pName[i] != '_') Name[i] = pName[i];
        else break;
    }
    return Name;
}

This should work

pawn Код:

new str[128], pName[MAX_PLAYER_NAME];
GetPlayerName(playerid, pName, MAX_PLAYER_NAME);
format(str, 128, "Full name: %s || First name only: %s", pName, firstNameOnly(playerid));
SendClientMessage(playerid, 0xFFFFFFAA, str);

This is one of many ways to do this

p.s.
Kevin_Warde would be Kevin
KevinWard_e would be KevinWard
K_evinWarde would be K
_KevinWarde would be
KevinWarde_ would be KevinWarde

I think that is what you wanted?