GBA: Issue drawing sprite object over background

Question

I'm trying to write a very basic GBA game - I'm coming from years of C# coding, but new to C and GBA programming.

I have been reading the tonc examples and am using the tonc lib. I have something that builds and runs, but I am failing to get a sprite to draw over a background. At the moment, the background renders ok, but the object over the top just appears as a glitchy pink square in the top left of the screen.

int main ()
{
    initMain();

    while(1)
    {
        vid_vsync();
        key_poll();

        updateMain();
    }

    return 0;
}

void initMain()
{
    // background
    memcpy(pal_bg_mem, backgroundPal, backgroundPalLen);
    memcpy(&tile_mem[0][0], backgroundTiles, backgroundTilesLen);
    memcpy(&se_mem[30][0], backgroundMap, backgroundMapLen);

    // character
    memcpy(pal_obj_mem, characterPal, characterPalLen);
    memcpy(&tile_mem[4][0], characterTiles, characterTilesLen);

    oam_init(obj_buffer, 128);

    REG_BG0CNT = BG_CBB(0) | BG_SBB(30) | BG_8BPP | BG_REG_32x32;
    REG_DISPCNT = DCNT_MODE0 | DCNT_BG0 | DCNT_OBJ | DCNT_OBJ_1D;
}

void updateMain()
{
    // character
    int x = 96, y = 96;
    u32 tid = 0, pb = 0;

    OBJ_ATTR *character = &obj_buffer[0];
    obj_set_attr(character, 
        ATTR0_SQUARE,               // Square, regular sprite
        ATTR1_SIZE_64,              // 64x64p, 
        ATTR2_PALBANK(pb) | tid);   // palbank 0, tile 0

    character->attr2 = ATTR2_BUILD(tid, pb, 2);

    obj_set_pos(character, x, y);
    oam_copy(oam_mem, obj_buffer, 1);

    REG_DISPCNT = DCNT_MODE0 | DCNT_BG0 | DCNT_BG1 | DCNT_OBJ | DCNT_OBJ_1D ;
}

I am no doubt making every basic mistake there is, but I have muddled myself along this far and just need a little bit of help to understand what I've done wrong here.

Thanks in (gameboy) advance!