Persistent

Persistent means continuing to exist or occur over a prolonged period

Merciful means compassion or forgiveness shown towards someone whom it is within one’s power to punish or harm

Tenacious means persisting in existence; not easily dispelled

Intermittent means occurring at irregular intervals; not continuous or steady.

Relenting means become less severe or intense.

Yielding means not hard or rigid.

Hence tenacious has similar meaning as persistent.