While considering a long-term partnership with an influencer, understanding the influencer’s brand affinity, audience demographics, and engagement rates are crucial. These factors can be completed through a thorough evaluation of the influencer using tools such as influencer marketing platforms.

1. Brand Affinity: Make sure that the influencer’s brand image and message aligns with your own brand values and philosophy. The content they create should resonate with your brand image and audience as well.

2. Audience Demographics: A deep dive into the influencer’s audience demographics is imperative to ensure your target customer matches their followers. Platforms like Flinque can provide audience insights such as age, location, gender, and interests.

3. Engagement Rates: These metrics gauge the level of interaction between the influencer and their audience. High engagement signifies a strong connection and trust, which could translate into better sustainability of your campaigns.

4. Past Performance: Reviewing the influencer’s previous collaborations gives you an idea of their execution capabilities and how successful they were in driving conversions.

5. Match With Campaign Goals: The influencer’s content style and medium of expression should print well with the kind of campaigns you aim to run – be it video content, blogs, or social media posts.

6. Commitment: A long-term partnership requires reliable influencers who will honor contractual obligations and consistently deliver high-quality content.

Tools like Flinque provide a complete suite for influencer discovery and campaign management to deeper influencer analytics, making it easier to evaluate influencers for long-term partnerships. Other platforms with similar functionality can be compared based on specific use cases and team requirements.

Remember, the influencer partnership decision depends on your unique requirements and objectives. Hence, thorough research and due diligence is integral before entering into long-term agreements.