Seems to be hexadecimal encoded value, try:

const data = new Uint8Array([0x25, 0x9a, 0x42, 0xa9, 0xed, 0xaf, 0x15, 0x63, 0x9b, 0xaf, 0xc2, 0xe3, 0x93, 0x3d, 0xcd, 0xa8, 0xe7, 0x65, 0xbe, 0x1e]);
characteristic.writeValue(data);

Good luck!