Zabbix user. It's the default user
root@WRT000:~# ps w | grep zabbix
16300 zabbix 1316 S /usr/sbin/zabbix_agentd -c /etc/zabbix_agentd.conf -f
16302 zabbix 1316 S /usr/sbin/zabbix_agentd: collector [idle 1 sec]
16303 zabbix 1324 S /usr/sbin/zabbix_agentd: listener #1 [waiting for connection]
And it have a valid user shell:
root@WRT000:~# grep zabbix /etc/passwd
zabbix:x:53:53:zabbix:/var/run/zabbix:/bin/ash