| sedulous |
| self-willed(adjective, tough, firm, determined, resolute) |
| unfailing(adjective, strong, firm, determined, steady) |
| wilful(adjective, determined, resolute, uncompromising, unshakeable) |
| adamantine(adjective, tough, firm, determined, resolute) |
| assiduous |
| devoted(adjective, strong, firm, determined, steady) |
| dour |
| energetic |
| industrious |
| loyal(adjective, strong, firm, determined, bold) |
| refractory(adjective, tough, firm, determined, unreasonable) |
| stern(adjective, terrible, determined, resolute, rigid) |
| strong-minded(adjective, determined, fixed, courage, serious) |
| unchangeable(adjective, set, hard, stiff, resolute) |
| unshakable(adjective, mean, strong, hard, tough) |
| bulldogged(adjective, tenacious) |
| faithful(adjective, strong, firm, determined, bold) |
| ruthless(adjective, appearance, strong, harsh, terrible) |
| strong(adjective, tough, firm, energetic, determined) |